The book "Light" of 1931 cites a Witchtower study from June 1927, where the reasoning behind the 1919 date is given.
I tried to obtain a copy of this, but Beth Hell refused to give it saying we have all we need in the current litterature to be spiritually strong.
I ended up being quizzed by the Elduhs for months, it was obvious that they were trying to find out if I was apostate.
At that point I still really believed it all, but this was a major nail in the WT Coffin.
HB