Leendert, I think the 423rd Base Unit (or 423rd CCTS -Combat Crew Training Station-Heavy) was the training unit at Walla Walla.
http://usafunithistory.com/PDF/0400/...ASE%20UNIT.pdf
Look at the accident list here:
https://www.aviationarchaeology.com/...y/Mar1945S.htm
Note that some B-24 accidents involved B-24s at Walla Walla. Also look at all the P-63 accidents and how most were assigned to training or base units.
Or this one:
https://www.aviationarchaeology.com/...y/Feb1945S.htm, which shows P-63 42-69086 crashing on 2/27/1945. It was assigned to the 423rd BU at Walla Walla.