I'm installing GFCI outlets in my 1950s era existing kitchen counter wall boxes. The metal boxes in the wall have a number of holes in them that I thought were for standard green grounding screws, but are a bit too big for that. If I tap them to 1/4-20 threading and use a hex/serrated flange 1/4-20 bolt to secure my green grounding pigtail to the box, is that OK with current code? My understanding is that as long as the screw is not a sheet metal screw and has no other function than to secure the grounding wire I should be OK.
Since I have only front receptacle mounting access to inside of box, and clear access to 2 of the holes I can see due to wiring obstruction, it would greatly ease my task if I could mount my grounding screw to one of those 2 locations.
Update 6/2022: Harper was right - after vacuuming out a bunch of debris from the inside of the box, I found what seem to be 2 standard ground screw holes hiding toward the inside sides of the box. It does seem like it is a 2-gang or at least fairly wide box with a 1-gang-opening mud ring on the front. Now I've just got to get a U-joint to be able to use one of the screw holes.