Here are a couple of older threads about these screws:
https://67-72chevytrucks.com/vboard/...hlight=9419728
https://67-72chevytrucks.com/vboard/...hlight=9419728
Looks like the part number is 9419728 and they may be designed to work with backing plates that had threaded nuts welded to them. The NOS backing plates don't appear to have those nuts, and NOS mirror sets appear to have come with smaller self-tapping screws. If that is the correct part number, then it looks like these screws may be available on eBay for a different application.
https://www.ebay.com/itm/33432101450...Bk9SR6DhqsqbYw