Here are some photos of the bolt thread measurement and nut across-flats measurement.
(In my previous post I forgot to double check and made the basic mistake of letting the knife-edges of the caliper jaws slip into the bottom of the screw thread, many apologies for muddying the waters.)
I guess any M8 flanged nut (preferably self-locking) will do.
eg the nuts on UK Amazon with code B07KKKX219
I can’t post the url because the word ‘flanged’ without the ‘d’ is forbidden by the forum editor and is a part of the link!
Sorry but nothing to do with the fixing problem but just a shear delight to see a proper Vernier in use not a digital or dial one, just one that makes you eyes squint to read it, 0.486 in, 12.22 mm if my 67 year old eyes in form my grey matter, cheers Rob.