I think Walla Walla Army Air Field at this time was a combat crew training center for B-24 crews. I believe P-63s were used to train the B-24 gunners. One type of P-63 (called the RP-63) had extra armor and carried no guns. The B-24 gunners actually fired frangible bullets at them.
https://www.airspacemag.com/military...t-me-57581298/
See page 1 and page 13 about the RP-63
http://www.pmlaa.org/newsletters/2003-03.pdf
For the curious, Joseph F Cotton's MACR:
https://catalog.archives.gov/id/90902611
The other type was a standard P-63 and the gunners used camera type devices on their guns to photograph the attacking fighters as they aimed and "fired" their guns. It is possible that the P-63s were of the second type and were part of the base training unit at Walla Walla.