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My guess is somebody tried to force the wrong size thread in the hole. I should have mentioned that electrical stuff uses a different thread series than most unc applications. You should browse the electrical section of your hardware store for odd sizes.
If you get a tap to repair threads, get what is called a bottoming tap. They aren’t tapered so you can clean up threads all the way down a hole.
Or you can get a small thread repair file. Each file will have several thread pitches on it. My hardware store carries UNC, UNF, and Metric sizes.
They will repair bolt threads as well.
But you can make a cleaning tap out of a hardened bolt. Get a grade 5 or grade 8 bolt and use a dremel tool to grind a couple vertical slots. It’s an old shade tree mechanic trick.
Cast iron is pretty soft. I’ve even used a dental pick to clean up small threads. Just needs patience.
For years I’ve had an eye out for adjustable dies, that would make bolts of different diameters with the same pitch. A century and a half ago they were common, but I think they all wore out. That’s why you see some trades use different diameter screws all with the same pitch. Electricians use 6-32, 8-32, and 10-32, so with one adjustable die amd a tapered tap you could do all the sizes you needed. Whitworth had a similar series. They can mount a switch, but the cover on it and close the box up with a couple tools.
Ceiling fan boxes are 10-24 because they hold more weight, and so you can’t fasten a fan to a box that isn’t rated to hold the weight., I have no idea what they do in metric countries, but you might be surprised at grandfathered sizes just like camera mounts are still 1/4-20 everywhere.
Gas threads follow a similar pattern.