Maybe it's just my tired old eyes but I'm not seeing anything special about the ring. It's a simple black collar correct?
As far as a nylon insert - make one. It's not rocket surgery.
7/16x14 OD x 5/16x18 ID. However....if you're looking for the 'fit' of the pin into the insert as Huebler has then you'll need to bastardize a tap.
Buy a cheap HSS 5/16x18 tap and OD grind the threads from .3125+ down to .3000". That way the pin will fit snugly.
Make a 'blank' insert, just the OD threads. Then install it into the shaft.
Drill & tap the insert after it's installed, that way, if you've trued the shaft in the chuck, the interior threads of the insert will also be centered true to the shaft.
KJ, I would have though it was just black also, but unfortuately, no. If you look close, it's looks like black and brown, the ring on the butt has the same kind of stripped look. Thanks for the advise on the insert. If I can't find anyone with a premade insert, I will try that. I apologize for the picture, it is hard to see.
If I look close, I can see a weave, so my guess is a phenolic linen. At this point, I'm not sure if it originally looked that way, but as I said, the ring on the butt matches.
Looks like the single black linen , which wasn't all black like the double black linen . Might call Atlas and ask if they have any laying around . They had a clearance on it a few years back .