Gun Sight Type N-8A "Retiflector"
This is a WWII-era Gun Sight Type N-8A "Retiflector" as used in flexible gun positions mounted on top of 50 cal machine guns in US Army Air Force bombers such as the B-17, B-25, and B-26.
Our friend Laurent offers a good description of this and other WWII-era gun sights at his website Aircraft-Gunsights.
This example is in reasonable condition. The flippable sun shade is no longer present and the bulb is not installed. But it is a fine specimen from the era. See spec sheet in photo gallery as well as a photo of the sight mounted in a B-17.