As you probably already know, Long Beach CA assembled cars have an "A" on their door post serial number tags (example, K512-A002456) but run the same serial number sequence as cars from Willow Run. Also, Long Beach cars have a 3 digit paint number that should translate to a lacquer paint rather than enamel (if original factory paintwork) because the plant did not have the high-temp baked enamel paint lines (that according to articles in AUTOMOTIVE INDUSTRIES magazine).
Check out the info on Long Beach operations in KFOCI Handbook Version 4.0.