Certainly looks like an M20.
The one I had was a 1937, proper pre war model, so slightly different cosmetically. Most fun bike ever and by far the slowest bike I ever had.
In the display classifieds in the back of the Green 'un and Blue 'un, companies like Pride and Clarke and Gamages sold ex-WD M20's. The deal included a bike - often brand new, old stock. They also supplied a wooden packing crate which also held a spare engine. They threw in either a "storm coat" or ex- services great coat, motorcycle dispatch riders (DonR) boots - any form of helmet would have been exta, they may have thrown in an old beret if pressed. The cost was for an all inclusive £5 for the deal.
They were on offer well into the 60's. Or at least still being advertised as available.
I remember in the mid 50's, my uncle buying an ex-WD M20 to travel to wherever it was he worked. I don't think commuting as such had been invented yet.
He roared up outside our house dressed in "slacks" and a very floppy shirt. He collected my heavily pregnant aunt who had to sit side saddle on the foam pad, and zoomed off again.
All speed related adjectives have been adjusted for my pre motorised perceptions.
