Thank you for the input so far, gentlemen. Much appreciated.

@ Don R., thank you for the info on the springs. That is very useful. Also the link from Garvin's forum is fantastic, with the technical info/tips.
The leather seal is a flat disk, about 5.5 mm thick. I will pick up a 1.1" leather cup type piston seal today, which will probably seal better, but reduce the swept volume. We'll see.
I might have to find a thick piece of leather, and a metal pipe with 1.1" inner diameter, to make a new flat type seal.

@Lakey, thank you for your very interesting comments.
The piston also has a "star" stamped on it, near the skirt. I think it is original BSA, but the rings might indeed have been a gunsmith's work?
And is it best to push the pellets deeper into the tap, or is it fine to just drop them in and let the air pressure do the rest?
Unfortunately, I don't think that Knibbs or Protek sell the paired springs anymore. (Knibbs: "BSA Round Section Wire Mainsprings (Pair) Part No. STD20 ***No longer available" - Protek: only non flat section springs on their website).

@Baz, the overall length of the piston (rod not included) is about 114 mm (4.49").
The "height" of the head is about 18.5 mm (0.73"). Would be interesting to know if it is a standard size, or longer to accomodate the rings.









When in the cylinder, the bronze rings still have "gaps". Perhaps not the best fit?


The "star" is on many parts of the gun. I believe this was the case with these guns anyway? Not sure what it means (must have a look in the BSA book!).
The "0" is also under the barrel. Does this mean that the tap is original to the gun?