Throughout most of WW2, US APOs were assigned by location, rather than by unit. Thus, the same APO could have been used for several units in the same location. There are several lists of APOs available on-line. Doc
All units of the Second Bomb Division used the same APO. Here is a useful post from the ArmyAirForces forum with a listing of APOs for the AAF units in the ETO. AAF Stations in the ETO on 1 June 1944
My father was at Ramgarh Training Center for part of his enlistment. I pulled out his letters home. His address on many was something like HQ HQ RTC. Good to know. Thanks! Never occurred to me to use that.