This is a Surefire IR Filter flip cap. It fits 6p sized bezel items. I had it on a hk forend light. I got it used when I got the light. There is a smudge or something on the side. I imagine its from muzzle flash when it was on the hk light prior to my purchase. This filters out all light except IR to aid night vision devices. It would turn your SF light into an IR light. When this is on a SF light NO visible light comes out from it. IR is invisible to the human eye.