Having just read this last night...

The P-51's in america were P-51A, P-51B, P-51D, and so on whereas the same
plane for the brits would have been name, Mustang Mk I, MkII and so on.

The brits used roman numerals for their spitfires to denote different models
such as Mk VI and Mk VIII, but when they got to 20, they gave up and went to 20
and and so on.

Nothing was "pure" so the P-51C was exactly the same as the B, but built in a
different factory and, although the americans used the B or C, the brits called
them the same "mark".

Neal