Included is a picture of our gearbox with studs. It requires special standoff nuts.
It may be because our bike was in North Africa and has a non standard outer cover.
It might be a modified M20 gearbox but the same gearbox shell was used before the war with a different cover, which has shouldered nuts to fit inot the holes in the cover. The give away is het selector shaft in pre war gearboxes it is much fatter than in the WD gearbox. I guess it's about an inch in diameter.