I've been going through the 145 Sqn ORB and have found myself stuck on deciphering what is listed in the Aircraft Type & Number column. I'm certain that the 3 digit numbers are the last 3 digits of the aircrafts serial but the single letter before them has me confused. The letters are A, B, L, T, N and M. A is there multiple times as A.324, A.339, A.321, A.502 and A.286. The aircraft serial is sometimes listed later with a different letter before it.
I have attached an image of the ORB which is downloadable from the National Archives website.
Does someone know what the letters mean?