Gun Sight, Reflector, Mk IIIA 7A/1313 RAF RCAF GJ3

Here is a fine example of the Reflector Gun Sight Mk IIIA, 7A/1313.  The Mk IIIA, aka Barr & Stroud model GJ3, was adopted by the RAF as the standard turret and free gun sight for their aircraft.

The Mk IIIA consisted of a detachable lamp unit and an upright sight head. The lamp unit is removed by depressing a small button on the bottom of the housing, revealing the bulb and circuitry. The brightness of the bulb was adjusted by the gunner by rotating the rheostat knob. The bulb illuminated the reticle which was projected through the lens onto the glass reflector from below. The glass reflector is contained within the housing above the lens. The reflector housing has a flip-up sun screen, which is engaged by turning a small knob on the side. 

This example is likely made for aircraft of the Royal Canadian Air Force, noting the remnants of the white label on the housing.  It is in very good condition given its age. The glass is intact as is the housing and the power cord.  Measures ~ 4 inches tall, and 3 inches by 3.5 inches wide.