THE STORY OF HOW , IN 1946, TRIUMPH TURNED a Lancaster bomber generator engine into a Manx GP winning racer is often told, and the Triumph Grand Prix is justly famous. There was, however, another manufacturer that raided their wartime stores for a motor. And, unlike the Triumph, which was essentially a prewar twin with a lightweight alloy barrel and head, the Douglas “Mark” series not only had a new engine, but a radical new frame and suspension system too. Douglas had mostly built inline flat twins prewar, with the exception of a shaft-drive transverse flat twin, named the Endeavour and a more conventional lightweight two-stroke.
During the war they stopped making motorcycles despite having built 25,000 for the military during the First World War, and made other things for…