Both pure and revised versions of the LXX from the first century and earlier contain forms of the divine name rather than kyrios. So whatever form of the LXX the NT writers had access to, the external evidence is that it contained the divine name rather than kyrios.
However early manuscript evidence of the LXX (i.e. before the second century AD) is quite meager so I do not believe it is a sufficient basis to claim that NT writers would have only had access to copies that had the tetragrammaton or a variant thereof. The possibility of the use of kurios in the Greek OT before the first century AD is certainly raised by such external witnesses as the citation of Exodus 9:3 LXX in Aristobulus (second century BC) and the citation of Deuteronomy 7:18-19 LXX in the Letter of Aristeas 155, (second century BC), and the likelihood increases substantially when we take into account the fact that kurios was used frequently as a title for God in Hellenistic writings from the same milieu, such as the Wisdom of Solomon (first century BC), 2 Maccabees (first century BC), Joseph and Asenath (first century AD), etc. (cf. Wisdom 4:17, 18, 9:13, 2 Maccabees 2:8, 3:33, etc., Joseph and Asenath 6:2-7, 12:1-13:15, 21:10-21, etc.). This is particularly apparent in the case of Philo of Alexandria (first century AD), who not only used kurios in OT allusion but also directly commented on it, e.g. hierais graphais kuriói onomati ho ón ... prosagoreuetai de hé men poiétiké theos ... hé de basiliké kurios in De Abrahamo 121 (cf. Exodus 3:14-15 LXX), to onoma kuriou theos aiónios ... hé men gar kurios kath' hén arkhei, hé de theos kath' hén euergetai in De Plantatione 85-86 (cf. Genesis 21:33 LXX), ho kurios ón hós kai blaptein dunasthai ... kai estai kurios emoi eis theon in De Plantation 89-90 (cf. Genesis 28:21 LXX), etc. Moreover, internal evidence also suggests that the tetragrammaton was not original to the LXX at least in certain instances. One striking example is Leviticus 24:16 LXX:
MT: "He that curses (nqb) the name of Yahweh (yhwh), he shall surely be put to death". LXX: "But he that names (onomazón) the name of the Lord (kuriou), let him die the death".
4QLXXLev b (which is an exemplar of the OG or "pure text" as you put it since it is not representative of the kaige revision) in fact has the oral form Iaó in Leviticus but it seems unlikely that the original translator would have used it since this would have done exactly what Leviticus 24:16 LXX prohibited -- it would have named the name of the Lord. And at least in the judgment of Lust the use of Iaó in this text is a secondary addition to the manuscript. Another example concerning the OG is the blunder in Daniel 9:2 LXX, where the nonsensical prostagma té gé in MS 88 clearly reflects the prior presence of YHWH (cf. dbr yhwh in the MT) in the Greek text. Yet the older Pap. 967 has kuriou here, suggesting just as easily that YHWH is recensional than the alternative.
As for the NT, the internal evidence supplied by the Pauline letters is quite decisive imho that kurios was original to Paul and not a form of YHWH. The replacement of kurios with YHWH obscures rather than enhances the author's argumentation at least in these cases (which are all found in the NWT where "Jehovah" occurs):
Romans 10:9-13: "That if you confess (homologésés) with your mouth, 'Jesus is Lord (kurios),' and believe in your heart that God raised him from the dead, you will be saved (sóthésé). For it is with your heart that you believe and are justified, and it is with your mouth that you confess (homologeiti) for salvation (sótérian). As the Scripture says, 'Anyone who trusts in him will never be put to shame'. For there is no difference between Jew and Gentile -- he is the same Lord (kurios) of all (pantón) and richly blesses all (pantas) who call (epikaloumenos) on him for (gar, indicating a logical relation), 'Everyone (pas) who calls (epikalesétai) on the name of the Lord (kuriou) will be saved (sóthésetai)".
Altered text: "That if you confess (homologésés) with your mouth, 'Jesus is Lord (kurios),' and believe in your heart that God raised him from the dead, you will be saved (sóthésé). For it is with your heart that you believe and are justified, and it is with your mouth that you confess (homologeiti) for salvation (sótérian). As the Scripture says, 'Anyone who trusts in him will never be put to shame'. For there is no difference between Jew and Gentile -- he is the same Lord (kurios) of all (pantón) and richly blesses all (pantas) who call (epikaloumenos) on him for (gar, indicating a logical relation), 'Everyone (pas) who calls (epikalesétai) on the name of Jehovah will be saved (sóthésetai)".
Here Paul uses Joel 2:32 LXX as a prooftext supporting his argument that the confession of Jesus as Lord brings salvation. Notice the use of gar to connect the OT intertext to his argument which in the previous sentence he referred to Jesus as the same "Lord" (kurios) of all (pantón) and who blesses all (pantas) who call (epikaloumenos) on him. This uses the language of intertext (i.e. the words pas, epikalesétai, and understanding sóthésetai as a form of blessing) in his own argument, which supports his main assertion that if you confess that Jesus is Lord (kurios), you will be saved (sóthésé). This latter word also reflects the use of Joel 2:32 LXX. Throughout kurios is the central word, for it is part of the declaration that brings salvation according to the scripture. Unless Jesus is understood as "Jehovah", the argument would make no sense when the name is interpolated in the scriptural citation.
Romans 14:8-11: "If we live (zómen), we live (zómen) to the Lord (kurió); and if we die (apothnéskómen), we die (apothnéskómen) to the Lord (kurió). So, whether we live or die (zómen te apothnéskómen), we belong to the Lord (kuriou). For this very reason (eis touto, indicating the logical relation), Christ died (apethanen) and returned to life (ezésen) so that he might be the Lord (kurieusé, notice that this is a verb) of both the dead and the living (i.e. everyone). You, then, why do you judge your brother? Or why do you look down on your brother? For we will all (pantes) stand before God's judgment seat. It is written: 'As surely as I live (zó),' says the Lord (kurios), 'every (pan) knee will bow before me; every (pasa) tongue will confess to God' ".
Altered text: "If we live (zómen), we live (zómen) to Jehovah; and if we die (apothnéskómen), we die (apothnéskómen) to Jehovah. So, whether we live or die (zómen te apothnéskómen), we belong to the Jehovah. For this very reason (eis touto, indicating the logical relation), Christ died (apethanen) and returned to life (ezésen) so that he might be the Lord (kurieusé, notice that this is a verb) of both the dead and the living (i.e. everyone). You, then, why do you judge your brother? Or why do you look down on your brother? For we will all (pantes) stand before God's judgment seat. It is written: 'As surely as I live (zó),' says Jehovah, 'every (pan) knee will bow before me; every (pasa) tongue will confess to God' ".
Here Paul quotes Isaiah 45:23 LXX in support of his argument that Christ (who died and returned to life) is the Lord over both the living and the dead. Interpolating "Jehovah" here breaks the connection between Christ's lordship over the "living and dead" and the lordship of the "Lord" (kurios) over "every knee and every tongue", as well as obscuring the connection between Christ "coming to life" in order to become Lord (kurieuó) and Christians who "live for the Lord" (cf. the same verb zó in the quotation in which kurios declares that he "lives"). The verbal kurieuó "to be Lord" is a clear sign of the originality of kurios here. The interpolation of "Jehovah" in the NWT in the places indicated here thus obscures the sense of the text.
1 Corinthians 1:28-31: "He chose the lowly things of this world and the despised things, and the things that are not, to nullify the things that are, so that no one may boast (mékaukhésétai) before God. It is because of him that you are in Christ Jesus (en Khristó Iésou), who has become for us wisdom (sophia) from God, that is, righteousness (dikaiosuné), holiness and redemption. Therefore, as it is written, 'Let him who boasts (ho kaukhómenos) boast (kaukhasthó) in the Lord (en kurió)' ".
Altered text: "He chose the lowly things of this world and the despised things, and the things that are not, to nullify the things that are, so that no one may boast (mékaukhésétai) before God. It is because of him that you are in Christ Jesus (en Khristó Iésou), who has become for us wisdom (sophia) from God, that is, our righteousness (dikaiosuné), holiness and redemption. Therefore, as it is written, 'Let him who boasts (ho kaukhómenos) boast (kaukhasthó) in Jehovah' ".
This is another instance in which Paul uses the language of an intertext in his argument. He paraphrases Jeremiah 9:23-24 LXX which supplies additional language used in his argument: "The Lord (kurios) says this, 'Let not the wise man boast (mé kaukhausthó) in his wisdom (en té sophia autou), and let not the strong man boast in his strength (en té iskhui autou), and let not the rich man boast in his wealth (en to ploutó autou), but let him who boasts (ho kaukhómenos) boast (kaukhasthó) in this (en toutó), the understanding and knowing that I am the Lord (egó eimi kurios) that exercise mercy, judgment, and righteousness (dikaiosunén) upon the earth, for in these things is my pleasure' ". What is relevant here is that being en kurio "in the Lord" is what may be boasted about and the antecedent for this in the argument is Paul's statement that you are "in Christ Jesus" (en Khristó Iésou), who is the focus of the preceding sentence (notice, for instance, that in Jeremiah one must not boast in one's wisdom, whereas Christians may boast "in Christ Jesus" and he is their "wisdom"). The thread of the argument is broken when Jehovah is interpolated here, unless it is understood that Jehovah and Christ Jesus are the same thing.
1 Corinthians 2:15-16: "A spiritual man, on the other hand, is able to judge the value of everything and his own value is not to be judged by other men. For (gar, indicating the logical relation), 'Who can know (tis egnó) the mind of the Lord (noun kuriou), so who can teach him?' But we have the mind of Christ (noun Khristou)".
Altered text: "A spiritual man, on the other hand, is able to judge the value of everything and his own value is not to be judged by other men. For (gar, indicating the logical relation), 'Who can know (tis egnó) the mind of Jehovah, so who can teach him?' But we have the mind of Christ (noun Khristou)".
Here Paul quotes Isaiah 40:13 LXX (tis egnó noun kuriou), applying the language in a way that indicates that he regarded the "mind of the Lord" as the "mind of Christ". The disjunct emphasizes not the contrast between the mind of Christ and the mind of the Lord (which for him are the same) but between needing to be TAUGHT the mind of the Lord and HAVING the mind of the Lord. That is his overall point in the chapter, that one has the "hidden wisdom of God" not by philosophy (which is taught) but by having the "spirit of God" which itself instructs (cf. 2:7, 11-12). The insertion of "Jehovah" here obscures the point being made, unless again it is understood that Jesus Christ is Jehovah.
Several other examples come to mind. In 2 Corinthians 3-4, Paul uses the example of Moses's shining face which had to be veiled to contrast the difference between freedom in Christ and the Law, such that the veil is over the minds of those who read the Law whereas it is lifted for those who are in Christ (3:14). Paul goes on to say that "with our unveiled faces we reflect like mirrors the brightness of the Lord (kuriou), all growing brighter and brighter as we are turned into the image that we reflect, this is the work of the Lord (kurios) who is Spirit" (3:18). The NWT inserts "Jehovah" in both places here. But Paul clearly has Christ in mind, for he goes on to refer to the veil over the minds of unbelievers who cannot "see the light shed by the good news of the glory of Christ" which is "the glory on the face of Christ" (4:4, 6). The "glory on the face of Christ" (prosópó Khristó) is the clear parallel to the brightness on the "face of Moses" (prosópon Móuseós) in 3:7. An even clearer example of arbitrariness is the references to the "cup of the Lord" (potérion kuriou) in 1 Corinthians 10:21 and 11:27. Both occur in discussions of the Eucharist (cf. 10:16-17, 11:23-26) and have in mind the cup of wine representing Christ's blood. Yet the NWT in the first passage has "cup of Jehovah" while in the second has "cup of the Lord", when they clearly refer to the same thing. The Society apparently replaces kurios with "Jehovah" in the first instance because v. 22 (é parazéloumen ton kurion "Or are we inciting the Lord to jealousy") directly follows which paraphrases Deuteronomy 32:21 LXX: "They have incited me to jealousy (autoi parezélósan me, cf. v. 19 which says that kurios ezélóse "the Lord was jealous") with that which is not God". So because 1 Corinthians 10:22 has a biblical allusion, the NWT uses "Jehovah" there and extends the name backward into v. 21, hence "cup of Jehovah". Yet they cannot be consistent and have "cup of Jehovah" in the next chapter because not only is there no intertext there, but the same verse refers to "body and blood of the Lord" and the previous verse refers to "the death of the Lord," both clearly having Christ in view. Of course, Christ is also in view in the parallel in ch. 10 for the "cup of the Lord" in 10:21 has as its antecendent the "cup of blessing" that bears "the blood of Christ" (10:16). This perfectly illustrates how inserting "Jehovah" in the NT creates all sorts of problems in the text.
The central issue in this discussion however should always be the fact that no textual authority exists for "Jehovah" or the tetragrammaton itself in the NT, and thus the NWT not only departs from all the textual witnesses in its interpolation of "Jehovah" in the NT but it also did so on an arbitrary basis. Yes, it is indeed possible than in some cases, the actual copy of the LXX used by an author of an NT book may have had the tetragrammaton in it, but this possibility should not be used as a basis for translation. There is zero attestation of the tetragrammaton or variant thereof in any NT manuscript, regardless of date. Every instance of "Jehovah" in the NWT-NT contradicts 100% of the extant Greek witnesses of the text in question. The Hebrew "J versions" are not witnesses of a text that originally had YHWH since all of these (like the NWT) are based on the same manuscripts that had "Lord" instead of the name. The way the Society cites these "J versions" in the critical apparatus of the NWT Reference Edition alongside legitimate textual witnesses imho is disingenuous, but that is a separate matter. What I would like to point out is that BECAUSE we have no textual witnesses of the original use of the tetragrammaton in the NT, its insertion in the NT is necessarily done on an ad hoc arbitrary basis. Did all books utilize a form of the LXX that had the divine name, or only some of them? The Society assumes that the name appeared in the version of the LXX used by MOST writers of the NT (as the name is subsequently interpolated in 20 out of the 27 books of the NT), yet the internal evidence from Paul indicates at least that he knew and used kurios. Do you insert "Jehovah" only in direct quotations from the OT, or do you also do it in looser allusions? How about discussion in the immediate context of the quotation? How about in cases where no allusion to the OT exists at all? There are examples of each of these in the NWT. Even if the name did originally appear in the autographs of the NT, it is impossible to tell precisely which instances of kurios are really examples of YHWH and which originally belonged to the text. And I believe the above examples show that once you begin to insert "Jehovah" where you think it ought to be, you potentially create new problems with the text.
