I think that they start out as something like #8 or #10 self-tap screws. But with enough on/off cycles, they get stripped out and there are two or three different kinds of screws in mine
Somewhere, I've seen that somebody sells an insert rod with threaded holes so that you can use (all of the same) machine screws. It wouldn't be hard to make such a thing yourself.
Anyhow, just take a few different-sized sheet-metal screws and try them in your holes. Maybe they're still all the same. I prefer the hex-headed ones that you can use a nut-driver on.