I coulda swore I posted on this thread but I don't see it... anyway, if you are still looking for a B50 wheel go here....
I used to spend a lot of time over there & the forum has some really good guys & resources. there are a couple guys that moderate the BSA board that have stuff... we traded a lot of things back and forth. ping Gordon Gray and Steve Ericson. there is also a Triumph board, etc.
as I recall the wet frame singles all have the same rear wheel.... B50, B25, and even the Triumph TR25W... that would be years from '71 and newer