OHC BSA.

[Hampshirebiker]

Can any of the learned crew give more information on this BSA? So far, I believe it to be a 1938 factory prototype of 500cc. It could also be an M24 Gold Star frame.
I would appreciate some more background &, particularly, who designed it. I believe Herbert Perkins was BSA chief designer at that time. Bert hopwood (who took over in 1949) has a picture of it in his book; but no text relating. The caption says late 40s, but that is wrong IMHO. He possibly included it to add a bit of beef to his design portfolio.
Google does not bring up anything; apart from the photo.