FWIW, given those are standard threads with larger heads than usual for their thread, it would be relatively easy to make new ones, if you have a lathe (or can improvise one). You'd get 2-56 round head screws, standard head, in brass, machine a head with a matching recess, and silver solder the screw into the head recess, then cut the head off the stock, touch it up, and cut the slot. Alternatively, you'd get flat head screws and a suitable oversize washer, countersink the washer to match the screw head profile, and optionally extend the slot across.
Probably a cost prohibitive process if you need thousands (though that starts to approach cost effectiveness for a screw machine run), but if you need a couple screws and have the tools, not a huge project.