i recently bought an ft3 in great shape. some of the silvering on the focusing screen appears to be rubbed off or marked. there is also a black ring visible in the viewfinder. is this water damage? the body is superb with minimal to no brassing. are these marks indicative of age, as the camera was launched in 1977? p.s. can the screen be replaced in a CLA, given that it is non replaceable?