If you removed a sight from those holes - the little screws would provide the diameter and threads per inch - is most common now-a-days for scope bases and aperture sights to be provided with either 6-48 screws of sometimes 8-40 screws - unless some Bubba got at it and forced a wrong size or thread in there - then you will have no idea what you need, without re-tapping the hole. Plug screws are available in both sizes - and, of course, some rifles seem to want metric sizes - that is a different nomenclature - need to know outside diameter of screw and distance apart of the threads - so can be like M6-1.0 or M6-1.25 (I'm making that up - I do not know if those are real sizes). In either case, once you get the thread correct, then have to deal with the length - often with a file.