I like the BMW picture---pre 1954 R51 or R67 or R68----In the late fifties I HAD 1954
R51/3 (first year of full hubs) $400 used in Germany and Steib S-350 sidecar $50 attached to 500cc NSU Konsul...which I threw away.
With solo gearing faster in 3rd gear than 4th. Very reliable bike, traded at 230,000 km---Plunger frame ok sidecar but, but a little ruff solo---bike purchased with 16,000 km, at end of bike boom when used bigger bikes had little value.