Gyro Horizon Indicator, British RAF Sperry Mark 1B Japanese The MC-125 Tuning Unit is

The MC-125 Tuning Unit is a component of the WWII-era airborne communication system Receiving and Transmitting Command Set SCR-AJ(and AL)-183

or Mach Speed Indicator

This is a Machmeter

The knob would rotate the face of the dial while the pointer remains fixed to set the present field elevation

for example

Gyro Horizon Indicator, British RAF Sperry Mark 1B Japanese The MC-125 Tuning Unit isHere is a British vacuum driven Gyro Horizon Indicator Sperry Mark 1B as used by aircraft of the Royal Air Force and British Fleet Air Arm during the 1940's and 1950's. This one has a manufacturing serial # incorporating "52", likely made in 1952. Measures 4. 5 inches diameter by 6. 5 inches long. The instrument looks terrific and would make a wonderful display piece, as long as you "Handle Like Eggs"!