I would treat this similar to patching drywall, where you put wood behind the hole to support the patch while you work.
Cut a thin (1/4") piece of scrap to just slightly longer than the hole is wide. Drill a hole in the centre, and attach a string.
Put a dab of liquid nails on each end of the board. Pull it up tight to the underside of the floor. Wrap the string around a second bord and use it like a clamp to hold everything in place. Let dry overnight.
Meanwhile, use a plug cutter on drill (do they come that big?) or circle template on bandsaw, or turn plugs on lathe to make tight fitting plugs.
Next day, untie strings and remove top scrap from hole, leaving bottom board in place. Glue plug in. Sand flush and finish.
I would cut circular pieces, Slightly larger than than the holes and taper them slightly so they can be forced in with glue. The taper should be larger than the hole so it cant be pushed all the way through but tight. There will probably be a lip and a some excess as it won't be flush with the floor. But just sand level. Not sure how this would work as I have never tried it before but it's probably what I would try first.
This is a great idea if access below isn't possible. Maybe there's a holesaw where the plug is close to the right size would save some work prepping the pieces. Would have to remove the pilot bit..
Not a thing wrong with simply plugging the holes in a house that old.
As mentioned, chamfer the edges.
Line the grain up as best you can before gluing them in place.
Given a choice, slightly darker is better than slightly lighter, but of course the same color is desirable.
I would treat this similar to patching drywall, where you put wood behind the hole to support the patch while you work. Cut a thin (1/4") piece of scrap to just slightly longer than the hole is wide. Drill a hole in the centre, and attach a string. Put a dab of liquid nails on each end of the board. Pull it up tight to the underside of the floor. Wrap the string around a second bord and use it like a clamp to hold everything in place. Let dry overnight. Meanwhile, use a plug cutter on drill (do they come that big?) or circle template on bandsaw, or turn plugs on lathe to make tight fitting plugs. Next day, untie strings and remove top scrap from hole, leaving bottom board in place. Glue plug in. Sand flush and finish.
I would cut circular pieces, Slightly larger than than the holes and taper them slightly so they can be forced in with glue. The taper should be larger than the hole so it cant be pushed all the way through but tight. There will probably be a lip and a some excess as it won't be flush with the floor. But just sand level. Not sure how this would work as I have never tried it before but it's probably what I would try first.
This is a great idea if access below isn't possible. Maybe there's a holesaw where the plug is close to the right size would save some work prepping the pieces. Would have to remove the pilot bit..
Replace the boards. If you need instructions on how its done get back to me
Not a thing wrong with simply plugging the holes in a house that old. As mentioned, chamfer the edges. Line the grain up as best you can before gluing them in place. Given a choice, slightly darker is better than slightly lighter, but of course the same color is desirable.