There is no problem with mounting 39mm screw mount lenses to other cameras with the same mount, unless you run into a Leica collector who accuses you of heresy. As for putting one on a KR10, you will have a problem because the body of the KR10 is too thick to permit focusing at infinity. So even if you found the right adapters, you would only be able to use it as a macro lens. This can be a source of confusion with the early Zenit SLRs, which have a 39mm lens mount, but different lens to film distances than the rangefinders. Given that the USSR made the same model lenses for both, it is easy to get the wrong one.