Just to clean up a few Errors..
2) Plus, ever notice that the KS-750's frame is one big piece of stamped steel and the R-75's frame is a bunch of bolt-joined sections?!?!? Welding technology was in it's infancy and the tubular steel frame was still not up to off-road rigors
Actually, the KS750 frame is mainly oval section tubes. The Headstock is made from two stampings welded () to give strength.
The R-71 was already obsolete during it's production and was the last flathead BMW ever designed!!! Olny about 3,500 were ever made.
The initial R75 prototypes were flat head. Its only because of the superior KS750 OHV design that BMW changed the configuration.
3) The KS-750 and R-75 weren't designed concurrently. Zundapp (who probably copied theirs from the Gnome & Rhone AX2 design or that other Belgian factory I can't remember right now) rolled out their rig well before BMW even had an R-75 assembly line.
The AX2 is a French design, the Belgian bike you are thinking of is the FN m1000. True, both BMW and Zundapp studied captured machines. But early drawings of what was the then known as the KS700 (with and engine based on the KS600) were already in progress by 1940.
The Germans wanted BMW to start producing the Zundapp design, but BMW insisted they could make their own version in a timely manner. The R-75 was the first motorcycle with hydraulic brakes.
Both R75 and KS750 deployed a cable operated front brake and chair and rear wheel hydraulic brakes by ATE
5) No post-war KS-750's or R-75's were made probably because the factories were destroyed and there were Allied enforced displacement restrictions on motorcycles. I read that there were about 100 civilian model R-75's made in 1948 from leftover parts, though.
Zundapp delivered approx 475 KS750 between 1945 and 1947.