Hi Dohunter. BSA was allocated the code M47 (Midlands manufacturer number 47) & as there were several BSA facilities they were all subdivided further into 'M47A' (BSA Small Heath), 'M47B' (Redditch), 'M47C' (Shirley). I think there may have been a 'D' as well, but I'm speaking from memory now without the books in front of me - no doubt you get the point anyway. The Shirley plant marked early No4 rifles with the letter 'B' usually followed by or immediately above the date eg. B 1941. They started using M47 at some point in 1943 & later in the same year went the whole hog & started marking rifles M47C. Thus 1943 manufacture rifles can be found marked B, M47 or M47C, depending when during that year they were produced.
The lack of an 'S' on the cut-off block may simply be indicative of the fact that a replacement rear sight was required to be fitted during the sniper conversion process. I have owned a couple of 1943 BSA T's that lacked the S like yours, & they were fitted with Savage made rear sights which had certainly been on them for a long time, & which I suspect were fitted when the rifles went through Holland & Hollands - perhaps because the rifles arrived fitted with just battle sights, which were not acceptable on snipers' rifles.
Hope this helps to flesh things out a little for you. I'd still buy those books from Brian anyway though!!
ATB