I've got some somewhere. If no one else chimes in by tomorrow, I'll start hunting for them. Don't really want to let them go, but at least I can get detailed measurements for you.
From memory:
The thread is British, but I don't recall offhand if it's BA or BSF (or BSW). They're about 3/4" long (but 1" would work OK). Screwdriver slot on the outside end, inside end is turned down to minor diameter of the thread (ie smooth) for perhaps 3/16". Should be a felt collar that goes on there too. TRF claims they have the felt, under P/N 7H4429
PS, Welcome to the asylum. One tip: It's usually better to start a new thread with a new question, even when it's related to the original question. Usually, you'll get more responses that way.
PPS, Perhaps I'm mistaken about the British threads. TRF says that a JN2108 will work for the jam nut, but JN2108 is 5/16 UNF. I still don't think that is correct, but it has been a long time since I had to fiddle with those dang posts.