My B31 has a frame the same as that. I can only presume someone got hold of an M20 frame, then re-stamped it at some time. It had competition plates on when I got it, so it probably started life as a plunger. Stamped up as a ZB31, from '48. I'm actually after a rigid B31 rear frame if anyone's got one to swap, I'd like to make it all B31.
