That is it.Wow, I missed that link somehow. Thank you @Scuba Cobra and @Sevenrider860
I guess this is it, although a description is kind of vague. I can't think what else it could be though, so it's probably it.
BUT the OMS valves might not be worth adding these caps onto to make singles.
I am not going to go through the soft parts bit by bit but, the OMS HP seats and burst disks and crush washers are all crazy expensive. So parts to overhaul your valves - something that is customarily done roughly every 5 years at hydro time may exceed the price of a new DGX manifold. Which has far cheaper service parts. Might be cheaper and easier to start fresh.
OMS cap alone = $17
Brand new DGX valve = $39
Do the HP seats, crush washers and burst disk add up to >$22? Not positive but its going to be close.