The NTs were manufactured in Dallas Texas while the NAs were manufactured in Inglewood Ca. For the game's sake the P-51C has the Malcolm hood.. The B has the standard canopy.. In this sim in order to differentiate between modded and stock versions of the P-51D20 the modders designated the mod version as P-51D20NT while the stock version remains NA.
You can use any P-51 B skin in a C and vice versa, although there are different skins for both but there are subtle differences if you are a perfectionist and want accuracy. The P-51 Ds use a different skin because the shape of the planes is different.
Note the line at the base of the canopy on the skins for the P-51C. That is because the Malcolm hood slid back. The C skin is also what you would use on the MkIIIs as well.
Note the absence of the line onthe B skins.. that is because the B has a birdcage and opened.