Back in the 90s I self published the little booklet: "Daughters of the Tower". Which traced all the offshoots of the Society since the death of Russell in 1916. Among them was a short history of the Standfast movement. I'm in the process of updating it. Currently working on the Standfast chapter which could be a book of its own. So far some 42 pages.
The Stand Fast Bible Students Association were formed in 1918. They accepted the 7th Volume and separated from the Society. Believing the door to heaven was shut and the harvest was over, they pretty much gathered together doing nothing but waiting to be called home to heaven. While they waited they bickered about little things until they split and split and split into oblivion.