M72 toolbox
Comparison of R71, CJ750 and M72 frames from Lutz Kraus
Here are some frame pictures. The black one is Chinese frame of recent production. The rusty one is a Soviet frame, and the brown one is a German pre-war frame. In general, they have the same dimensions.
The BMW frame has the pumpholder and the steering lock. It is not reenforced. I have many more pictures and details, but this will be too much. Also, I don’t know the technical terms to describe the small details. :-(