What is the mirror stop
The Mamiya 645s are notorious for broken mirror stops. The mirror stop is a small plastic piece designed to hold the mirror at a certain angle (around 45 degrees) when it’s down. The mirror stop consists of a plastic piece (the one that breaks), a spring, and a tiny screw that is used to adjust the angle of the mirror.
Calibrating focus
Adjusting the screw will tilt the mirror slightly up or down which results is moving focus. Turning the screw clockwise will tilt the mirror up, and move the focus backwards. Turning the screw counter-clockwise will move the focus forwards. To calibrate focus you need an objects that’s easy to focus on and a piece of ground glass to stick in your camera where the film usually sits – or a (super expensive) digital back. You also need a tiny flat head screwdriver and a lot of patience.