I've got one of these bikes with the sidevalve engine. They are based on the BMW R71.
The generally accepted history of these bikes is that the Soviets got the licence plus tooling for the BMW R71 under the Molotov/Von Ribentrop pact in 1939 and produced it as the M72 up to about 1957. Most of the Soviet heavy 4 stroke bikes are descended from this Model. The Ural factory in Siberia transferred the tooling to Red China who then produced it in the Nanking Aircraft factory up to about 1996. The picture shows the Chinese developed OHC model called the M1S.
My bike us under restoration and tuning at the moment and will be finnished in feltgrau. I have a friend who has one in unrestored in PLA green. They are no less reliable than an old BMW and cheaper to fix. Same goes for Soviet bikes, of which I have 3. WW2 Renactors use these as a replacement for the BMW R12 although only a few requisitioned R71s were used by the whermacht. I have one downloaded picture of an R71 used by the German mountain troops in north Norway.