BSA was a major industrial combination in UK, a group of businesses manufacturing military and sporting firearms, bicycles, motorcycles, cars, buses and bodies, steel& iron castings etc etc.
In November 1919 BSA launched their first 50 degreeV-Twin, Model E, 770cc side valve (6-7 hp) motorcycle for the 1920 season. The machine had interchangeable valves, total loss oil system with mechanical pump and an emergency hand one. Retail price was ?130. Other features were Amac carburettor, chain drive, choice of Magnet0 or Magdyno, 7-plate clutch, 3 speed gear box with Kick starter functions.