there's a brass pipe protruding from the back of the lens, made of relatively thin brass, with three cuts in it. This has to catch the inside focusing ring-thing of the mount on the camera, when you set the lens to infinity and line up the red dots. Further, this has to move IN into the lens a bit when you push the lens on the camera for mounting.
Either this is rotated and cannot catch the three holes on the inner mount on the camera, OR, due to old lubricants it is stuck and when you push the lens after line-up of dots, it cannot move into the lens. The alignment you can check approximately by eye before you put the lens on; the moving-in you can check simply by trying to push this pipe thing into the lens by hand, or by putting something flat on it and pushing. It does need some force (there's a strong spring involved) but should go in by a pressure that you can exert with a few fingers.
