From BMW Tech

BMW M2B15 is an aircraft engine. In accordance with the ideas developed by Max Friz, Martin Stolle developed a 500 cc side-valve Flat engine, referred to internally as the Type M2B15 and offered for sale officially as the "Bayernmotor". The engine was tried out by various motorcycle manufacturers. Starting in 1920, Victoria (motorcycle) of Nuremberg began to use the "Bayernmotor", and other manufacturers such as SMW also fitted it.