Airspeed / Mach Speed Indicator, US Air Force Type ME-4
This is a Type ME-4 Air Speed / Mach Speed Indicator with Maximum Allowable Indicator as used in the Cold War and Viet Nam War-era's US Air Force Fighters F-100 Super Sabre and the F101 Voodoo. See images from their flight manuals.
The indicator displayed airspeed from 60-850 knots, and mach speed from 0.5 to 2.2 mach. The knob, when rotated, adjusted the air speed index pointer along the perimeter of the airspeed scale. The red maximum airspeed pointer index is set by accessing an adjustment screw, which is concealed beneath a cover screw on the back left side.
It is in fine condition given its age. The serial number prefix is AF-55, indicative of a year of manufacture for a US Air Force order in 1955. The glass is show a little haze which could be readily cleaned if opened. Measures 3.25 inches diameter and ~5 inches deep.