You can bet the basic pressing started life on another model. My '53 Parts List Super Flash supplement lists the tank complete as 67-8362. The filler cap is 66-8351 so looking for tanks with this cap and accompanying split pin 35 707 plus filter 29 7962 could turn up something that will be close enough for now. M20 and early C range are a a good call for starters. Early B range worth a look.
Pre 1949 parts catalogues listed in the Forum Literature Section cover all models from the whole BSA Range, so while a bit of a chore looking, all parts for all models are listed and illustrated in one place.
Swarfy.