Things are starting to make sense - especially the fact that the derailure is a Nuovo Record.

The bolt is just a standard hex head? That may be, but maybe the nut is a rectangle.

It seems I remember that either the nut or the bolt was a goofy shape that you couldn't substitute.

Let me check my stable. I have a couple (actually, about four) of 1970's Raleigh 10-speeds. I should be able to pluck a reference for you.

If, however, you could get by with a standard fastener, then drill and tap a new hole as D*Alex wisely suggests. It is not difficult to do. In fact, any high school shop class should be able to do it for you.