Wow, thanks for such an in depth reply!
Indeed it is an older 25mm piston mk2 - mid 80’s I think? I fitted a tinbum kit and gave it all a good clean/polish and new seals but that’s as far as I’ve gone with it. Already shoots nicely, but was rather confused by the lighter pellet results.

As far as I know the transfer port is standard (but would be lying if I told you I knew anything about that stuff...). What would it be normally? I will have to look for that thread!

Just added accupells to my list thanks!