Gunsight, K-13 Compensating Sight

The K-13 Compensating Gunsight, made by Sperry, is a electrically-powered gunsight designed to compensate for altitude and airspeed.  Typically mounted on the rear housing of a 50 cal machine gun from a side/waist gun position in the aircraft.  Used in Boeing B-17 Flying Fortress and Consolidated B-24 Liberator bombers during WW2. It was a late war innovation that used the aircraft altitude and speed with the direct correlation of where the weapon was pointed to calculate the reticle. The aircraft altitude and speed was entered on the rear of the sight.

Here is an interesting video describing how the K-13 functioned.