Thank you trajectory for your advice on pellet seating.

I will test the gun in the following days, with a few setups:
- new 28 mm leather cup type piston seal
- a number of springs (Protek advised a Meteor spring, which I have. I have also picked up another spring from a Diana. And then I will order an Airsporter spring)
- with and without the piston rings. Hopefully they will come off easily (without damaging them); otherwise I might have to leave them on.
- I will try to make/have made a spring guide to slide over the original one, as the springs mentioned above all have a loose fit over the spring guide

About the history of this gun: I bought it from someone who found it in a gunshop not long ago. That's all I know unfortunately.
The trigger block has more wear to the metal, and had a different/more faded finish than the rest of the gun. I wonder if it was on this gun originally...
It will never be a collector's piece, but hopefully it will turn into a good shooter.

Cheers