Engine with JB20 engine number is a B20 250cc side valve model from 1938 season. It would have been fitted into a frame starting JB20.
JB21 is also an engine number but for a B21 250cc ohv sports. It would also have been fitted into a JB20 frame.
The JB20 frames were fitted with B20 250 side valve engine, B21 250cc OHV sports engines, B22 250cc Empire Star engines and B23 350cc side valve engines.
Expect a relatively low production number of around 1200 each for the B20, B21 and B23.