If they’re #10 (US) screws, they’d be 4.8mm major diameter and 24 or 32 threads per inch, so something like M4.8-1.06 or M4.8-0.78. If M5-0.8 thread half way, it sounds more like 10-32.
If you’re outside the US, that might be why the previous owners resorted to the ugga-dugga. That will (probably) have wrecked those holes for either their factory pitch or whatever the owners used. You might consider getting a 5MM drill and a 6mm hand tap. You might have fair luck with 10-32 screws, depending on how hard they are to get in your country.