FBU 4EVER! From Norway, joined Jan 2001, 998 posts, RR: 7 Reply 3, posted (8 years 11 hours ago) and read 6367 times:
The U.S. uses a different system by adding a letter to the designation instead: P-51B = RAF Mustang Mk.III,P-51D = RAF Mustang Mk. IV,P-51K = RAF Mustang MK. IVa.