My late Father's war surplus BSA WM20.
The BSA WM20 was part of BSA's massive wartime contribution to Britain's war effort. 1937 was its first year of production that would last until the late 1950s. The "W" in the model designation signifies that it was specifically-deisnged to be a military bike (just like Harley's wartime WLA). Designed to be simple, rugged and reliable, it was a side-valve (flathead) 500cc air-cooled single with a cast-iron top end. It had a rigid frame and BSA's pre-war girder front end. Everything on the bike was extra-heavy duty for military use. The M20 was designed by Val Page as a heavy-framed sidecar rig, so the engine had tons of low-end torque. When sent to the British Army for testing (without the sidecar), they didn't like it saying it was heavy, slow and had poor ground clearance. But it was saved by its toughness, reliability and ease of maintenance and repair. Of course, BSA was the world's largest motorcycle maker at the time, and also a huge defense contractor, and that probably helped. In the end, the Army, the Royal Navy, and the RAF placed huge orders for the M20, along with Sweden, South Africa and India.