These are support brackets (bands), that fit Remington T-rex Bullpup PCP airgun. There is a non-bullpup version too.
It will support and stabilise the barrel, which will improve the accuracy. I think this is a must have attachment if you wish to have the long barrel fixed and parallel to the cylinder.
Material: black 3D printed strong plastic.

It's fixed by one grub screw, which is plastic and will not mark the surface.

You have a choice between just a band (12 mm) or 50 mm bracket with side picatinny (21 mm) rails.

Please, tighten the screw carefully observing the gap between the barrel and the cylinder to be even along the length.
Contact me for variations if you need any.

NOTE TO EBAY: 
We have checked that this item is not prohibited under your Firearms, ammunition, replicas and militaria policy. This is not a gun, a pistol, a rifle, a frame, a barrel or a receiver. It does not infringe your stated Firearms Policy. This item is for use with a particular pellet airgun only is just an accessory. It won't fit any other gun or firearm and doesn't breach any seller's rules.