This is an original butt pad for the Reximex Mito PCP airgun.
The pad is made of grey soft rubber and black plastic.
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.

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.