Looks like there are two kinds, stainless and black.....did Ford use different thread sizes over the years? I hope not! I'm not sure what year or model the wheels are that the PO put on....maybe even from 1974 Bronco options, for all I know.
I've found some various sources for the screws and it seems much easier to find the stainless than the black. So far, eBay seems to be the best bet...most venders seem to think they are made of gold...