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
Resurrection after a few repairs and now waiting on a wash.