Thanks for the reply Adrie, avoiding trouble is why I am seeking advice. You will have to enlighten me as to what is indicating this is a 51-53 case. The case shown has A7/479 but there is no casting mark on the other half, that I can see.
The dual port head and barrel have all the right casting numbers so presumably someone blew up their cases and obtained replacements from the factory to make this all work? I got this with a 1950 G108 gearbox which I know is not correct but I have a BA that will do although marked for 1939. I have seen video from the previous owner of this motor running so I know it is possible to rebuild it to run but it may be that I have to find some more suitable cases I guess.
'51 SQ4, '56 NH, '56 VH, '36 VH500 Red Hunter, '56 SQ4 (basket case), '49 Sq4 (basket case), '48 A7 BSA Twin, too many other BSA's, Norton's, Triumph's, Ducati's and Japanese to list. Just crazed is all my problem is.