Not a BSA engine
I have seen it before but just can not place it
I think I have just about every book writen about BSA no mention of any side valve twin form the 30's or 40's
Lots of stories about the WB30 OHV twin and the total redesign with the 3 ranges , B sporting light singles, M heavy singles & C light weight commuters starting at 1935 and of course the A series twins.
I stand corrected , it is as Tomcat mentioned .
So ignore my post above.