OK, they are definitely M3 as people have said. I have been looking on
Ebay so I now know what I am looking for. One more question, does the spring go inside or outside the dropout? I suspect the outside. Thinking about it, does it really require a spring. What's wrong with a lock nut? (oops, that's two questions). I have had many bikes over the years and this is the first one I have had that has these screws. No screws came with the frame but thankfully the threads in the frame are clear and free. This is the tail end of the bike showing one side's dropout. I know there are no cables. When the photo was taken, I was just doing a dry assembly.