The sixrobblees deal was to call them and they could provide Borg kits. This all came up because of well...aw shucks...j-d...little ol'me. I was shopping for extended valves in Anchorage AK and went to a commercial truck tire shop. They couldn't help but called 6R's at their nearby location. That shop didn't have any kits on hand but another location did. I went there and the only kit they carried was for 10-hole Ford wheels. When I went to check out, I was shocked at how much less they were than the marked retail price. From there, a couple other OP's here wanted some and we found we could look the number up on Borg and order from 6R by phone. 6R's price kept going up. They were still less than retail.
Here's Borg's Dually Valve Page for Class C. The kit I got was DL1E. There's contact information in two blocks at the top of the page. Call or email, hopefully they can get a set to you.
The stems are solid brass tubing with a Shrader Valve and cap threads on one end, of course. Then at the rim end there's a thick brass flange, about the diameter and thickness of two nickles. Below that it's threaded for a nut. All brass, brazed assembly of the flange, accurate threads, valve core hasn't leaked. Synthetic rubber washers, one side with a raised portion to center on the .453" hole. Then a metal washer and hex nut. Because the nut is on the inside if the wheel, it's important to install with LocTite since it can't be adjusted later. The hobby shop crew at my local Navy base installed ours and they've been fine. Local Discount Tire installed the replacement I mentioned and it's OK too. Good stuff.
Here's a photo of a long inner wheel valve that got a mounted tire/rim assembly dropped on it. It's supposed to be STRAIGHT... What you can't see is how the valve core end was crushed right where a gauge/air chuck goes. It still did NOT leak!

Installation of the inner valve can require a little bending. The approved way is to slide two small (say 7/16") box wrenches down it and use one against the other to create the bend. Not much, just takes a tweak to get through the rubber "stabilizer" on the outer wheel and line up with the hole in the stainless simulator.