It looks like one but isn’t. They’re from a couple of nice chromed steel and leather chairs I’ve got, but I’m 4 bolts short. I’m going to try a brake nut tomorrow actually as it might work. Just kind of annoyed me I couldn’t work out what they’d be called.
Look like brake pivot nuts.
https://www.sjscycles.co.uk/brakes/shimano-brr451-pivot-nut-for-front-brake-27mm-8a0-2310/