Work has been slow, but the parts from the Countrystore/ Airgun Spares in the UK finally arrived. Most of the delay in shipping was a stop in Chicago where the order sat for weeks.
I purchased three OEM seals, two piston rings, one e-clip (that's all they had in stock), a Pro Elite spring, and a Titan spring to fit a Pro Elite.
The Pro Elite spring sent must be UK-spec because the wire is significantly thinner than my U.S.-spec spring. It had a wire diameter of 0.128" with 31 active coils vs. 0.148" diameter/34 active coils of my original spring. The fit was loose over the original spring guide. Estimated spring rate is 31 lb/in vs. 48 lb/in of my original.
Here's a photo of the original Pro Elite spring (top) compared to the replacement from the UK. The difference is obvious.
The Titan spring sent is much closer to my original, but still uses a bit thinner wire at 0.141". It has 30 active coils, is nicely made, but is still a bit loose on the existing spring guide. The calculated spring rate is 43 lb/in which is very close to what the original US-spec spring is.
My conclusion? - Re-use my old spring and have the Titan for a backup. It would require new spring guides, but I can have those made when/if I need them. The link for Chambers in the UK shows two possible springs for the Pro Elite, so possibly one of them might be US spec. I may try to contact them in the near future to see about that.
The seals shipped looked very nicely made. They snap right onto the piston, but aren't tight without being glued. They can be pulled off again with little effort. They are possibly a urethane material that is more flexible than the Maccari seals that I pictured earlier. Also, they have a parachute lip around the edge. It has a significant outward flare.