Tinkering with the little red motorcycle, discovering that some sand-like stuff had gotten past the fuel filter somehow, and was opening the carbs for a cleanout. Remove one screw, a very special screw, peculiar to Mikuni downdraft carbs, and dropped it.
Search the garage floor and find it under the wifes car. There is also a washer that goes with this one, and not just any washer but a very special brass job made to very special specs. That one had disappeared. Checked fit and determined the size. Turns out it matches the back end of a 9mm bullet case. Drill out the primer hole to 5mm and cut it off at the groove, and Voila! the new washer is easily identified as the specialty Mikuni carb part by being prominently stamped "9mm Luger".
Works fine. And yes, after the wife went out for some errand and I got a chance to sweep the floor, I found the original washer, so now I have a spare. Making the new one took less time than finding the old one. Of course.