Custom bored and PTFE-Electroless Nickel Piston Body with Apex Small -Tesla Small- Factory Seal and a set of 

Tough Kore breech seals.

Takes your stem no matter if FAC or UK spec.

This is from the Monolith Tune Days- Late 90s. One Only.

JM's Airgun Classifieds (delphiforums.com)