Generally different models had different Serial Number prefixes.
Not all letters were used. A certainly wasn't. It would just have been a 955.
The ones I know are as follows. I don't guarantee that the list is complete and my Serial Number reference book isn't within reach at the moment.
955C - 12A
955F - 45A
955H - 60A, 72A, 77A, 93A
955K - 31J, 48H, 53H, 57J, 61H, 64J, 71J, 85J.
955L - 08Y, 13D, 13X, 57M.
64J prefix changed from K to L from S/N 3994 onwards
85J prefix changed from K to L from S/N 4672 onwards