Thanks for your help
'78 R100/7 '69 R69S '52 R25/2
With an R12, BMW made a ton of them, but they were for the war effort (there are prewar civilian bikes, but they're a small minority). The bikes went through the meat grinder of the war on the losing side, and then any of them that were still good enough to be made usable were patched up over and over in the east bloc so that some farmer could have transportation to the market.
One problem are forged numbers, to try to make a mish mash of parts look like a single bike. I wouldn't worry about it if your customer isn't worried. Some forgeries are totally obvious, but there are a few that are well done. Basically, if the numbers match, I would simply presume that one or the other is a forgery. If the frame and motor numbers don't come up in the tool here, there's also the possibility that the bike was renumbered after the war in some country besides Germany to get it registered for the road.
The really big problem is a bike with a huge number of deep, difficult/expensive to fix problems. The frame is strong and twists rather than breaking, so there are a lot of bikes with bad frames, wheels that point in different directions, and are likely to ruin driveshaft rubbers and the splines in the front of the final drive because they don't line up. The crankshaft is a pressed together unit, but a lot of these have been "fixed" by welding the parts together. The forks, which are the first production oil damped forks have a weird design that depends on some small leather seals inside. People take these apart, find it difficult to deal with, lose parts, and then try to fit something else.