It's always surprised me no end that QR setups never took off over your side of the pond to any degree. The difference between using a QR vice and a standard one is like night and day when working on large, varied size or multiple things. QR really does become like a best friend after only a short while.
We've always been spoilt rotten in that regard over here. Whereas you chaps are spoilt with the truly large vices over there, we have the QR over here.

Pretty much anything above homeowner or basic grade vices from the main manufacturers would incorporate the Parkinson style quick release system.
I'm quite sure it's a lesson you'll remember though.

If you're ever contemplating slipping a pipe over a vice handle, it's too much. I'm quite sure there are plenty of us who either have, or even occasionally still do, use a vice to straighten something, but they have definite limits in that regard. Pressing, bending, shaping etc. are all processes which are far better served by other means.
Have you any other, better photo's of that nut? Hard to make out from that one angle how it's designed and what might constitute a possible repair. Getting one of those as a replacement part will be like looking for hens teeth.
If you're in one of the places where the older Record vices were far more common, you might be better off just looking for a proper QR vice and then just using that existing vice as a light weight use one or standard vice, depending on if or how you can repair than nut assembly. The proper, old style QR setup should be far stronger, (judging by the looks of that nut assembly and ACME? thread leadscrew on yours), than the setup used on yours.