I do not know about Rob Bowman, but I have read both Walter Martin and Ron Rhodes. Both have stated that the WTS are an off shot of the Adventist. Charles Russell joned in the 1834 was the invisible return of Jesus and 1874 was to be Armageddon . When 1874 came and went 1874 was the invisible return and 1914 was to be Armageddon.
There are other groups that follow Russell's teach, like the Dawn Students ( a break off because of Rutherford). But none as powerful as the WTS.