BSA Shirley used a single letter prefix followed by a four digit number during 1941 & 42, then in 1943 went to double letter prefixes but with the first of the two letters always being 'A' (AA, AB, AC, AD etc) until they changed over to five digit numbers at the beginning of 1944. The first run of 1944 had no letter prefix, the second 'A', the third 'B' & so on.
The 'latest' prefix combination I can remember seeing for 1943 is AX, but AY &/or AZ may well exist. I've also seen one rifle bearing a A? serial that was dated 1944, but I've only ever seen the one....(I think it was an AX prefixed rifle).
The changeover at the turn of the years 1944 to 1945 seems to be blurred a little. I've seen 4T's bearing P & R prefixes dated both 1944 & 1945.
I've never seen this written down anywhere, so it's purely anecdotal, from my very patchy memory.