John,
First off, have you looked here?
http://www.mcmaster.com
Often the screw is held captive by a small snap-ring, this is common on tripods, for example. One option would be to find a screw that fits the receiving threads, then file or grind off all but the end threads and add a snap-ring. Tedious, but doable. Destructive power tools help.
If you have a real hardware store handy, you can identify the thread and size by hauling the camera or its pieces in and trying out different screws. If they are really well stocked they may have what you need, but it's not likely. But you'll have a better idea of what to look for.
Another alternative might be to call or email Richard Ritter, he may have something suitable on-hand. He has a lot of bits and pieces of old LF cameras. Or else, he may know what to tell you to buy from McMaster.
If you can identify the size and perhaps add a picture of what needs to fit where, I might be able to come up with something.