I know this is "after the fact" but when taking things apart with small pieces (screws, pins, etc.) I always put a towel down on my work surface before removing any parts. This way if a screw slips out of my hand or drops loose from the thing I'm working on it will land on the towel & now bounce or roll off the table. Believe me, I learned this the hard way when working on small items. Good luck in finding your missing screw & with restoring your beautiful machine. You seem to have a ton of good help here on the board for resources & information.