Scotrod85,
The German system of proof marks can be very confusing concerning the cartridge used in a particular rifle. In spite of this, some cartridges are virtually not in question. I believe your rifle is one of these and Raimey's assessment is correct. There is normally no problem using American 25-35 Win. ammunition in a 6,5x52R rifle, except the point of impact of the bullet may need adjusting due to the slightly different load. Considering what we must go through to shoot some other German rifles, you are very lucky to have one you can use American ammo in.
Mike