oldskool: I once discussed in depth the 1914/19 chronology and selection information with an Elder that attended ministerial training school and was taught under a governing body member. Faced with the documentation that the 1914/19 chronology was incoherent, he immediately transitioned the conversation to the Jonadab "miracle". This point was communicated heavily to him during his training, and I believe it remains in the background as the "backup" evidence to 1914/19 JW leaders rely on to define their divine appointment.
The reality is no Jonadab miracle occured...
I have always understood the "Jonadab" reference was always pulled out whenever the WTS had made alliances with persons/entities who were not JWs themselves. The 'helper class' has often been used synonymous with the 'Jonadab class'.
I think that the significance of the 1919 date had a lot to do with the political and financial alliances that Rutherford made, during/after his jail term, that allowed him to continue with business when faced with the certain ruin that the 1918 court case had left the WTS in.
At the time that Rutherford and his cronies were jailed following their charge with treason, the WTs was in shambles. By the time that Rutherford et al were released in 1919, the Brooklyn property was facing foreclosure and the printing presses were idle with no money for the fuel to run them and no printing plates because the plates had been seized by the government.
However, something miraculous happened during the WTS' leaders' brief jail stint - somehow...the notion was born to publish a 'supplement' to the Watchtower magazine. Except, this magazine was to be published 'incognito' as though it had no ties to the WTS. The leaders of the WTS emerged from their jail cells with a mission - to publish the Golden Age magazine. And they did. With the financial help of some person who arrived at WT headquarters with cash taped to his torso under his shirt - enough cash to get the buildings out of trouble with the banks and to get the presses up and running.
The arrival of that cash came on the heels of a trip that Rutherford made out to California where, supposedly, he found a benefactor to put his plans of publishing the Golden Age into effect - http://www.jehovahs-witness.com/topic/126813/lyman-swingle-gb-told-me-what-would-bring-down-organization?page=2&size=20
By the time that the summer convention rolled around in 1919, Rutherford was in the position to announce the trumpet blast - the magazine that would herald in the 'new world'...the golden age of scientific discovery, etc. And along with Rutherford's brilliant announcement, came the support of his cronies. One of the supporters was William Hudgings - an inventor and scientist - who would give a sermon at the Ohio convention on how the mystical appearance of the number seven signified God's involvement in this wonderful occurrence.
Hudgings' entire sermon can be downloaded at https://archive.org/details/1919BibleStudentConventionReport.
...For instance, the prominence of the figure seven in connection with the whole trial has so protruded itself upon our attention that we cannot mistake that it is more than merely a set of coincidences. For instance, the trial itself covered exactly fourteen court days — seven of those days were devoted to the preliminaries and the prosecution, whereas seven days were devoted to the defense. At the conclusion of that period the brethren were sentenced and convicted — eight of them — and then in the most remarkable way the eighth was separated from the others and the sentence came upon only seven, on the 21st day of June. The eighth was sentenced exactly twenty-one days later — three times seven.
Then they were taken down to Raymond Street jail. None of us knew how long they would be there. They kept them there exactly seven days, and while there, the officers of the Society and both compilers of the Seventh Volume were placed on a tier of cells known as Number Seven on the North side.
At the end of the seven days they were removed to Queen's County jail.
Nobody knew how long they would be kept there. They were kept there exactly seven days.
And on the seventh day they were removed from there and taken to Atlanta on the 4th day of July (the seventh month), which day was being celebrated by exactly seven nations for the first time in history.
Hudgings then goes on to attribute other mystical events to this appointment:
In addition to the prominence of the figure seven in connection with their case, I wish to also mention some parallelisms suggested during the blessed week we were in Raymond Street jail together. First one brother would suggest something and then another, and by the end of the week we had some very interesting data compiled that I think has gladdened the hearts of the friends who have heard it. The Lord, in a most remarkable way, has shown a direct relationship between the experience of the feet members of the body and the experience which came upon the Head of the body nineteen centuries ago.
We know that the moon was pictorial of the Jewish dispensation, which came to a close in the days of our Lord. Even as the sun is a picture of the Gospel dispensation. There were great privileges that had been extended to the nominal Jewish house that passed away because they did not appreciate the Lord from Heaven, who had come. Even now we see that the Lord has cast aside the great nominal spiritual Israel because of the fact that they have not appreciated the Lord's Second Presence and co-operated in the carrying forward of the message now due. Rather, they have fought against it; even as they fought against our Lord and persecuted Him to His death. Now let us note some parallels.
Once a year the sun reaches its zenith in the heavens. Brother Russell suggests that the waning of the moon at the time of our Lord's crucifixion was suggestive of the passing away of the privileges that had been extended to that nominal Jewish house back there. Our Lord was crucified when the moon was at its zenith in the heaven. He was crucified on the 15th of Nisan at high noon, according to Matthew's account. "There was darkness over the earth from the sixth hour." That would be noon, because the Jews reckoned daylight from about 6 A. M. It was apparently at high noon, therefore, that our Lord's activities on earth were cut short, on the 15th of Nisan when the moon was at its monthly zenith in the heavens.
These seven representatives of the Lord's people, and you and I, and all of us as feet members, had our activities in connection with the harvest cut short on the 21st day of June, 1918, the longest day of the year, when the sun was at its zenith in the heavens. It was at that hour that the sentence was pronounced upon our seven representatives.
This matter is further emphasized when we look into the details of the circumstances as they occurred. Our brethren were convicted the day previous (June 20th), and the Court had announced that at noon on the following day he would pronounce sentence. About 11 A. M. the officers came to Raymond Street jail and took our brethren to the courthouse. Everything was arranged just before 12 o'clock for the judge to make his appearance in the court room and pronounce the sentence according to schedule. But 12 o'clock came, then 12:10, 12 :30, 12 :40, and the judge had not yet put in his appearance. He did not arrive and impose sentence until 1 P.M.
"Oh," you say, "I am sorry he spoiled the picture. Why couldn't he have gotten there on time?" I do not know why. He did not explain, but merely apologized for having been delayed an hour; and then he pronounced the sentence at 1 P. M. instead of 12 o'clock, — as announced. But, isn't it a fact, that all the clocks were running one hour fast all last summer? (Laughter.) And if he had pronounced that sentence at 12, it would have been one hour before the sun reached its zenith in the heavens, on this longest day in the year. The Lord permitted him to be one hour late in order that the sentence might come upon the seven representatives of the feet members of the body at exactly high noon, when the sun was at its very highest point in the heavens.
And then the sun began to wane, even as the moon waned back there at the crucifixion of our Lord, and showed the passing away of the privileges upon the nominal Jewish house. Just so, this pictures the passing away of the privileges of nominal Christendom down here. The picture is complete.
Let us see further: the 13th of Nisan back there when the Lord was crucified occurred on Friday. You look at the calendar and you will find the 21st day of June in the year 1918 came upon a Friday.
Furthermore, the Lord was found guilty the night previous, — Thursday, at a late hour. Our brethren, the seven representatives of the Feet Members of Christ, were found guilty the night previous, at a late hour.
I tell you we cannot say that these are merely coincidences and that the hand of God has not been in the whole circumstances; we cannot shut our eyes and say this is merely an incident in which seven or eight men were involved. It is something in which you and I and everyone as members of the feet of Christ are involved.
So...I don't think that a person can find a specific bible scripture that says the WTS was appointed as God's "slave" in 1919, but it wouldn't surprise me if there are some dusty old documents archived somewhere that indicate who actually appointed the WTS to "be in charge of their belongings" in and around the time of 1919. Someone who bailed out Rutherford and his publishing schemes after the WTS was left in ruins as a result of the jail terms that Rutherford and his buddies landed (which were miraculously cut short). Someone who really is that "Jehovah" that the WTS is accountable to. A Jonadab type person/entity.
Whoever it was that "appointed" the WTS to be their caretakers in 1919 won't be named in the bible - they will be behind closed doors shaking hands with the WTS. And I think that the hand shaking started when Rutherford was still in the Atlanta jail - shaking hands with people just like him who were being held as political prisoners. People just as interested as Rutherford was in making money and gaining power.