Radio / Radar Altitude Indicator ID-257/APN-22, F-8E Crusader, F-101, OV-1
This is a Radio (or Radar) Altimeter Indicator type ID-257/APN-22 as used in the Vietnam War era F-8E Crusader, F-101 Voodoo, and OV-1 Mohawk, among others. The AN/APN-22 radar set is a microwave radio altimeter designed to measure aircraft clearance over terrain. The knob at left turns the index marker to the designed minimum clearance height and provides a warning should the aircraft drop below that height. See page from the F-8E Flight Manual as well as an illustration of the APN-22 components. Measures 3.25 in diameter and ~6in deep.
It is in decent shape for its age. The index knob is missing, but a replacement should be easy to find. This instrument has an overhaul and repair sticker from Naval Air Station Pensacola (FL), of Q1 1970. The dial markings are clear and the glass and case are intact. See photos.