BSA M20 article taken from Classic British Motorcycles by Andrew Kemp & Mirco De Cet

BSA M20 article taken from Classic British Motorcycles by Andrew Kemp & Mirco De Cet
It's the whole write up as it appears in the book. Pic is from book also.

"When world war II began, BSA was Britain's major motorcycle factory, with the proud boast that 'one in four is a BSA'. The firm had supplied the armed forces in world war one, and as befitted a company that began as an armament manufacturer, had geared up to produce munitions as early as 1935. They would go on to become the biggest supplier of motorcycles to the armed forces, in the somewhat unlikely form of their model M20.
Originally launched in 1937, mainly as a sidecar model, the M20 used a 500cc side-valve engine in a heavy frame. BSA had been supplying the war office with a variety of models for evaluation and in some cases purchase, since the late 1920's. But following the failure of their 1935 submission, the 35-6, the company submitted the M20 in 1936, only to see this fail, too, owing to heavy engine wear. Resubmitted the next year, it passed, but only marginally, and a small batch was purchased in 1938.
The war office then issued specifications for a lightweight model, specially designed for service use, and BSA, along with the other major manufacturers, developed their own prototype. Although this was well received and a few were built, the official policy then changed to favour the machines that were already in service, who's reliability was well known, principally the Norton 16H and M20. Large quantities of the BSA model were purchased, until it eventually became the leading service model, although far from the most popular with its riders.
Heavy, bulky, slow and with limited ground clearance, the M20 had far from an ideal specification, but was rugged, generally reliable and easily repaired. Special fittings included a long, spiked prop stand for field use and a large headlamp, fitted with a blackout mask in many areas of operation. There were other detail changes up to 1942, when shortage of rubber lead to the replacement of rubber items such as hand grips with canvas fittings and footrests with simple metal ribs. A large air filter mounted on the tank and coupled to the carburettor by a hose was fitted for use in dusty climates such as the African desert. As a result part of the rear of the tank had to be cut away.

Simplicity itself, the M20's rugged side-valve engine had massive flywheels, a low compression ratio and soft valve timing which made it both forgiving and flexible, with plenty of low down torque, coupled to a standard BSA gearbox. The primary drive chain case was a sheet steel pressing secured by a host of small screws, but the exposed rear chain had to take its chances n the desert sand or northern mud.
Purchased for dispatch and escort duty, the M20 served in many theatres of war. Over 125000 were purchased by the armed forces. Finish consisted largely of applying khaki paint liberally, often including the engine, tyres and even the saddle, with different camouflage schemes in different countries.
Vast numbers of M20's were discharged at the end of hostilities, and after a new coat of paint were snapped up by a transport hungry public. However the model stayed in service in smaller numbers for many years - in some cases as late as 1971. The vast quantities built meant that ex-WD M20's are still in use in many parts of the world, and it is even possible to find that new spares are still available. The civilian model was manufactured until 1955, with it's 600cc cousin the M21 soldiering on until 1963, the last side valve built in Britain."