Here ya go. Is this what you are looking for?

This nut and special bolt is on several of my 1970's Raleigh ten-speeds. This came from a Raleigh Gran Prix with a Simplex derailure. The other Raleighs all have different derailures, but this nut/bolt seems common to some of them.

Note that the nut has a raised groove that fits into the slot in the dropout. It holds the derailure in place.