I was changing lenses on my 645 Pro and this piece was loose and laying on the mirror. It is plastic and about 5mm long X 3mm height X 2mm thick. As shown, it has a machine type screw on one end. It doesn't appear to be broken, but???? The camera still seems to function in that the shutter fires and the mirror swings. Any help would be appreciated. A diagram with exploded view might possibly give some direction.