This is the M61 Lens and Head Gasket. The Lens is constructed a scratch resistant hardcoated shatter resistant acrylic. It is 95% transmissive (most standard glass lenses are 92% to 94% transmissive).
These are for using the M61 Dropin in a Malkoff MD2 Flashlight and Malkoff-Valiant VME Head. They are not neccessary when using the M61 in a SureFire Flashlight.