Grab the tap as close to the hole as practical to be able to turn it with vice grips or even an adjustable might work. Hit the top of the tap with a hammer while trying to move the tap back and forth. Something around an eight ounce hammer is what I'd probably use; make sure the hits are in a...
Pacific air compressors had the best price when I needed a pressure switch for my forty-year-old compressor. I used their eBay store.
https://pacificaircompressors.com/
https://www.ebay.com/str/pacificaircompressors
I'd drill a hole above the keyway and just into it some. Drill size about key width. Then I'd remove the rest of the metal above the keyway with a die grinder with a bit sized to fit between the gear teeth. Drive a wedge in there and pull it off.
How old are your sockets? Are they 12 point? Twelve point tend to do that more than six point. The newer six point sockets contact further down on the flat of the nut, away from the hex point. This helps to prevent the socket sticking on the nut or rounding it over.
It's used for rotating equipment alignment with indicators. It's attached to the shaft or coupling with the chain.
There were quite a few styles of brackets out there; I had a few myself. Old school now as most alignment is done with laser equipment.
Used brackets like that into the late 90s...
I've had decent results on eBay. I make sure to eliminate overseas sellers and try to buy from sellers that sell commercial or industrial products for the most part. I try to get them with free shipping if possible.
27 lines, 128 characters of text per line, plus spacing on a 17" laptop with the font sizing and display size I use,1400x900. No problem reading it for me.