Machmeter, Kollsman 815BX 1968 Grumman Aircraft US Navy

This is a Machmeter, or Mach Speed Indicator, made by Kollsman part number 815BX. It carries a calibration label by Grumman Aircraft Engineering Corporation (GAEC) dated 1968, see photos. USN 33451 has been etched on the back of the case, perhaps the serial number of the aircraft from which is was removed (?). Measures 3.25in diameter and ~4in deep. It's condition gives clues of its age.   The dial markings are clear and the glass and case are intact.