Yesterday I finished the Improved Model... for now.
I am using a shallow piston seal from Protek.
As a leather buffer, I am using the cut out bottom of another leather 28 mm seal.
And the original (?) brass dished washer and screw.

The leather seal seems to work well. It isn't too tight in the cylinder, but when moving the piston up and down with a finger on the barrel, it creates good vacuum / air pressure.

Springs wise, I tried a few combinations, but settled on what was in the gun when I bought it: a counterwound set of a flat section spring, and a normal round section spring that has a smaller outer diameter. The flat section spring is at the rear (on the spring guide), and the slimmer one is in the piston. I have put one steel washer in the piston. And a bit more grease on the springs than the thin smear of moly that I normally use. This time I actually used grease from a Vortek kit. Don't know why, but it's thicker than moly and I thought it would work better. I think it helps against vibrations and twang, because the gun is now nice and quiet, and has a pleasant shooting behaviour.

Chrony results: nice and consistent, but velocity is lower than I had expected.
650 ft/s with 7.0 gr RWS.
My Light Pattern does 620 ft/s, but is a bit harsher.
Overall, I prefer the Light: the length, balance and stock suit me better.