This is an adjustable 3D printed butt pad for the Reximex Mito PCP airgun.
The pad is made of soft rubber-like 3d printed material, very nice burgundy colour.
Fitting instructions:
Simply tap out the 3mm pin at the bottom of the original pad. You could use a small nail for this. Pull out the original rubber pad.
Then refit the new pad in the reverse order.
Tap the pin back in to hold butt pad in place.
Adjustment is made by slackening the cap head screw (allen key is needed), slide to position and gently retighten the cap head.

Note to eBay:
This item is an accessory for airguns and is allowed to be listed. 
It's NOT a firearm part or accessory - Please don't end this listing in error.