To my eye the real proof is the Afrika Korps insignia on the tank. When you know to look for it, you can plainly see that some of the white paint that came from adding that to the bike relatively recently is over both the tan and the underlying gray paint extending NW from the frame roundel. That insignia was never on the bike originally, and you can see that it is in pristine condition compared to the aged parts of the bike.
To the uninitiated: The R35 was a prewar model 350cc single that BMW made, and which the Wehrmacht ordered in large quantities. They were made at BMW's plant in Eisenach, which unlike the Munich plant, was never bombed during the war. Eisenach fell into the Soviet zone after the war, and almost immediately the Soviets had them making more R35s from remaining spare parts. These first models are identical to the prewar bikes except that the serial numbers begin with a 2 instead of a 3 -- so you can imagine that there's quite a business in forgery. The East Germans continued to make these bikes until 1956, although with an increasing number of changes. Again, there's a big business in converting post war bikes to pre war bikes. BMW had to sue the East Germans to get them to stop using the BMW Roundel, at which point the East German company, which came to be known as Eisenacher Motoren Werke, switched to a similar red and white roundel, but with a 4 pointed star separating the quadrants.
There are similar difficulties with the other models I mentioned. A pre war civilian BMW is more valuable than the utilitarian war bikes, especially if they have been pieced together from literally dozens of sources, with nothing much really matching. A war bike with a real pedigree would also be worth more than, as the author of the emails above calls them, parts zoos.