It's a common item on German military optics. Here's my NVA 7x40DFs, thought by some to be the finest military binoculars ever made. The fitting on the right is for the light source. On the left is a port hole that lets you "charge" an infrared filter during the day. At night, twist a knob, and the filter is swung into the optical path. It's supposed to let you detect infrared light sources, although I've never tried it.
Here's the light, complete with battery box, on the binoculars. This light is also used on East German aiming circles, BC scopes, and other optics. It uses two AA batteries, although the battery box is big enough to hold a dozen batteries. It took me about five years of patient searching to track down the rig.
Here's the whole outfit. The East German unit is on the right. At the very left of the picture is the West German scope illuminator. A red LED, two calculator batteries, and a dimmer switch, minus all the industrial-grade hookup of the commie model.
