Yes Bob, you are correct. That's why the NWT or at least the New Testament part of it, is not a reliable translation of the Bible. It is a biased version that was made to fit the JW theology. Ask any current JW and they will tell you that the name Jehovah was added to the NT because it had been removed from the actual Greek texts. But this is based more on what they THINK happened then the actual facts show.
As you mentioned, there are over 5000 ancient Greek manuscript copies of the Bible available today. None of these contain YHWH, the four letters of the Tetragrammaton, in the 237 places where the Society has added Jehovah. Some of the ancient Greek manuscripts date from the 3rd and 4th century. Just refer to a copy of the Kingdom Interlinear Translation of the Greek Scriptures, published by the Watchtower Society, and you can see the actual Greek word Kyrios appearing there. The Society lists about 20 J references in the Interlinear Translation as proof that they are justified in replacing Kyrios with Jehovah. However, the J references that the society has listed are actually taken from more recent translations of the Bible. The earliest one having been published in 1385. Also the only consistenancy with their replacing Kyrios with Jehovah, occurs when Jesus or Lord appears to be divine.
For more information on this, I would suggest that you refer to the following website: http://www.tetragrammaton.org/. It is an excellent site on this topic.
Hope this helps.