Airspeed Indicator, 500 MPH Type D-6 PN 586S-025
This Airspeed Indicator is a WWII-era US Army Air Force Type D-6, Kollsman PN 586S-025, as used across a number of US Army Air Corps and Air Force aircraft during WWII, including heavy bombers B-17 and B-24, medium bombers B-25 and B-26, as well as light bomber A-20, according to the Maintenance Interchangeability Chart dated Dec 1943. It was also used in versions of US Army Air Force WWII fighters P-38, P-39, P-40, P-47, and P-51.
The red pointer is a "dive marker", which was to be set for the maximum permissible speed in a dive for the aircraft type in which it was installed. The indicator was illuminated by a small 3V lamp at the upper right corner once twisted to make contact within it socket. The socket and cap are present, but the bulb is not.
It is in very good condition given its age. The dial and glass are intact. Measures 3.25 inches diameter and ~ 2.5 inches deep. This D-6 design does not have a dive speed marker.