rThanks guys. After a few days trawling the internet, it's easy to see why there's not a " definitive" burman guide anywhere : what a convoluted puzzle!! B52/GB BA/CP AMC/ARIEL Aaaaaargh!
However, I now have some further numbers to conjure with : GB28C55 stamped on the outer casing ( which, characteristically, is , of course, the middle casing ) This would seem to be a March 1955 box from a Matchless G80S or similar. None the wiser, however, whether this might mean that it's more compatible with a Sq4......
Will keep an eye out for a g1-5 shell although i can't find any noticeable difference between the two shells.
Then, of course, there's apparently an issue with offset lugs......
It'll all become crystal clear soon, I'm sure