This is a spacer that you just pop over the end of the air cylinder (2nd picture) before you screw your air cylinder into the block of your Weihrauch HW100 - this is to protect you from over-tightening the air cylinder into the block and damaging the brass regulator. On some HW100's you may notice a very small gap between the block and the air cylinder (hold the gun up to the light, you will see the gap) - this is perfectly normal - BUT a lot of people think that there shouldn't be a gap there and proceed to screw the air cylinder into the block even harder to get rid of the gap. When you over tighten the air cylinder into the block, it can damage the brass regulator valve and/or the valve inside the air cylinder and cause power & inconsistency problems.