Stig. Some of the planes not delivered camouflaged from the factory from 1940 like P-40s and B-17s were in the process of being painted when designators would have been revised (ie. low-vis). I thought you might like this official account which includes SERIALS in the Maintainance Records!
Regards
Nick
https://media.defense.gov/2010/Sep/1...100917-040.pdf