PS I've found the number on the outer 66-7503 which seems to be the usual BSA way on being one number out (66-7502). I guess for a part before it is finished?

Still looking for the inner number. Ron

email (option): ronpier@talk21.com

Had a look at the ones put by for my GPO WM20 and WB30 and Ian is right its the rear chain guard mount which differs.

Left marked 65-7578 Right 66-7602

Rob

email (option): robmiller11(a)yahoo.co.uk

66-7525 was the outer cover on 1940 M23 and 1939 M23 M24, how did it differ from 66-7502 M20 & M21?

Is that the version with the shallow clutch dome?

Rob

email (option): robmiller11(a)yahoo.co.uk