The engine looks to be a Triumph 3/1 which is a 350cc side valve. Made from 1934 to 1936. And the number on this engine does not appear to be correct and has been re-stamped?

I have a 1936 Triumph 2/1 which is the same bike except the engine is a 250cc OHV.

The thing I think that's odd about it is where the engine number is stamped, never seen one protrude like that before.

The 5SW crankcases were widened similarly to that engine to accept the base of the larger 500cc barrel...Ian