I'm not sure how your camera is set up, but I have a F-1, 2nd version, and it's possible to put the prism housing on wrong and not have it fully seated. I would make sure it's all the way down on the camera. If that isn't the problem, and you see nothing wrong w/ the mirror/lens mount, I would send it back. I buy from KEH a lot, and usually have good luck, but they aren't perfect. The good thing is that they will take a return. Aren't their cameras warrantied for 6 months? You could always send it back to them to be repaired.