First letters is the aircraft type, second is the number of types in service, third is the maker and finally any sub variants.

The P-51D mustang would probably been something like

F = fighter

1 = (First) Model of aircraft in naval service

J = North American

5 = fifth sub-type (P-51, P-51A, P-51B, P-51C, P-51D)

Here's my F1J-5