Hi EdenOne,
Your post got me to looking at two editions of The Harp of God, 1927 and 1928. Although some great changes* took place between these two editions they both have a chapter entitled Our Lord's Return, and both agree on 1874 as the time of Christs return and contain chronology calculations on how that number is arrived at.
You mentioned a reversal in Watchtower understanding of Christ's return, at first being 1874, then 1914, then going back to 1874. I wonder if this perception is simply based on a typo of the publication date of The Harp of God. Your quote below says 1937, but I wonder if Harp of God was still being published in 1937. It was a book from 1921 thru at least 1928, but would Watchtower have moved on to other books by 1937?
However, there seems to have been another reversal, for 3 years later in the book The Harp Of God, they go back to 1874:
"Applying the same rule, then, of a day per a year, 1355 days after 539 AD brings us to 1874 AD, at which time, according to Biblical chronology, the Lord's second presence is due" and "It was in the year 1874, the date of our Lord's second presence (...) - The Harp Of God (1937) p.235, 240
I couldn't find the exact quote above in either of my editions on p. 235 or 240 (but it may be somewhere in that chapter), although there was some 1874 information on p. 235 of the 1927 edition and the same information is on p. 240 of the 1928 edition.
Scans:



For a smile notice the list of things that proved we were in the presence of Christ since 1874. We had inventions and developments such as typewriters, artificial dyes, and the Panama Canal. It's quite a list. Such enlightenments proved Christ had to be present. Such was the silly thinking of JWs then, but today they have moved on to different silly thinking
*The changes from the 1927 and 1928 editions of The Harp of God included...
1) The subtitle, "Proof Conclusive That Millions Now Living Will Never Die," was dropped.
2) Information saying that Charles Taze Russell was the Faithful and Discreet Slave (Faithful and Wise Servant) was removed from the 1928 edition.
“Without a doubt Pastor Russell filled the office…and was therefore that wise and faithful servant, ministering to the household of faith meat in due season.” (The Harp of God 1927 p. 239, par. 420)