Rubber piece that fits over the occular eye piece of the rifle scope, ø39mm, semi-stretchable.
This eyepiece is 75mm in length, with a concertina body of 6 pitches. This allows one with this rubber piece installed on the telescope lens, to self-adjust one's eye-relief.
This rubber piece presses against the bone and is soft enough to avoid gouging the eye.
This piece is useful for warding off extraneous light that comes in from around the ocular, to provide a more concentrated sight picture. This is especially useful if the ambient light happens to be bright and one would need extra focus on longer distance / small targets w/out needing to adjust the magnification.