I think that one of the reasons for the confusion over when the Watchtower changed over from 1874 to 1914, is because of the different theological perspectives of the two leaders, CT Russell and his successor, JF Rutherford. CTR was obssessed with the Gentile Times prophecy that supposedly ended in 1914. For as long as this date was in the future, CTR had a modicum of credibility. To CTR this meant that the end of the world as we know it, and consequently, with the belief that Christ came "invisibly" in 1874, that his generation was seeing the last days [1874-1914 conveniently made up 40 years which meant the "last generation"]
By 1916, when CTR died, this prognosis had clearly been discredited, and at first JFR scrambled about attempting to justify some aspect of CTR's expositions. JFR did this by extending CTR's datings to 1925, when again, the world as we know it was expected to end. When this again failed, JFR went completely off the rails and began a corrupt process of biblical exposition that saw him delving into some of the most bizarre forms of Bible study.
He left dating things well alone, and the year 1874, though still evidently being upheld by him, was never mentioned in any intelligent way in any of the literature scribed by him. JFR began to see himself and the organization he was reconstructing as the centre of the theological universe and in order to "prove" this, he plundered the Bible to find adequate allusions to support his absurdities. Ruth and Naomi, Mordecai, Job, Jonadab among others, were enlisted to show how he and his organization were thus "fulfilling" Bible prophecy, without the need to resort to dating things.
Much of what he was teaching was so utterly cockeyed, that the Watchtower allowed his books to quietly go out of print as soon as they ran out of their time. Thus, whereas CTR was all for dating things because these dates were still in the future, JFR had inherited a defunct sequence of dates, which, because he was unable to adapt to his new teachings, were therefore ignored.
Throughout his life, he had never once conceived his theology in terms of a 1914 invisible return of Christ. Remember JFR had other concerns on his mind. He refashioned the Watchtower organization into his own image by wrenching it away from the more genteel construction first done by CTR. The generaly introspective meditative formula introduced by CTR was replaced by an intellectually dominant set of regulations which now required the ponderous study of every pronouncement made by him to his following.
Whereas CTR had focussed on the "harvest" as his main theological concern, and the soon approaching cataclysm, JFR instead focussed on replacing what he saw as an overly dependent view of Jesus Christ in the organization. Thus, detirmined to "put Christ in his place", he adopted much of the Sacred Name theology, which was becoming prominent at this time, and invented this "Jehovah" perspective, which became a new obssession. He instituted an editorial policy in the Watchtower which would insist on the inclusion of this name in almost every paragraph.
Despising the preaching work himself, he instituted regular ministarions which saw the rank and file requiring to go from door to door, while carrying awkward phonograph machines [which evidently he sold to the rank and file for a neat profit] blaring his stentorian voice.
He refashioned the organization which he inherited into an anti-Catholic fascist system of belief, with his position at the pinnacle unchallenged. He undid the fledgling democracy instituted by CTR in the local congregations, by imposing a dictatorial method of control which saw all elected officials in the congregations disbarred from office, to be replaced by flunkies personally selected by himself and responsible to him alone. With an ironic twist of euphemism, he called this authoritarian control a "theocarcy".
By the time of his death, the Watchtower organization that he bequeathed to his successors no longer bore any tolerable resemblance to that founded by Russell. Dates, which were first manipulated into place by Russell, were ignored, with no adjustments made by JFR.
Thus the literature of the time, if depended on too literally, can be misleading. My own research is possibly faulty, but the last mention I can find for 1874, is the book "Prophecy" published in 1929, where this statement is found:
"The scriptural proof is that the second presence of the Lord Jesus Christ began in 1874". [page 65].
It must be remembered that if this is indeed the last reference to this date, it by no means indicates that belief in this date was done away with at that time.
It simply means that from then on, JFR totally ignored this date, while still adhering to it.
Someone once pointed out to me that the JFR booklet "What is Truth", published in 1932, mentions the date 1914, albeit with no theological implication or even any regulated analysis of what that date meant. JFR died, still believing [if he believed anything intelligent] in 1874.
It required the perverse genius of Freddy Franz, erstwhile "oracle of God" to reinvent the second presence doctrine, with yet another emphasis on the "Gentile Times" doctrine, which his mentor, JFR had refused to comment on.
As has been noted above, the "Proclaimers" book on page 133, did publicly admit that this change from 1874 to 1914 was effected as late as 1943. That date then must stand as the official date of transition from 1874 to 1914. Nearly three decades after the event.
For a fuller understanding of this subject, a good book to read is "Captives of a Concept" by Don Cameron. A book I have shamelessly plagiarized in this post.