Ithaca bought some gun makers ie Lefever Arms,Syracuse Arms,Union Firearms,Wilkes Barre Gun Co. This is according to Conley's book. I dont have Walt's book yet, Ill try to get it ordered soon.
Ithaca also built doubles into the late 1940's and the SBT into the 1980s. Ithaca was in operation longer than any of the other high quality American double gun makers. The only exception I can think of off the top of my head is Remington.