Here's a bit of info:
Cal-Aero / Polaris Contract Basic Flying Schools
Chino Field Letter Code A then O
Lancaster (War Eagle), CA Field Letter Code B
Both were listed as flying BT-13s
Historical Notes
When BFTS#2 closed out of Lancaster, CA (War Eagle Field) the Polaris Academy performed contract Basic flying training for USAAF cadets. The British field letter code of "BL" was changed to "B" as Cal-Aero (Chino) was an "A". In 1944 basic training at Chino was cancelled and some assets were transferred to Lancaster. Therefore Lancaster had a mix of letter codes of both A & B.
In mid 1944 Cal Aero's Mira Loma Academy Contract Primary school (Oxnard) re-located to War Eagle (Lancaster) and replaced Polaris Academy contract basic.
Info taken from Flight Training Field Fuselage Codes website.
http://www.fuselagecodes.com/id1.htmlWikipedia also notes that the War Eagle Field training organization was the 18th Flying Training Detachment, for whatever good that does. I wonder if there might be a historical society in Lancaster that might be helpful.
Sorry that I can't answer your question for certain.