Helicopter Dual Tachometer Kollsman 981BX-8-023 H-21 Piasecki

Here is a dual Tachometer for use in military helicopters, indicating RPM for the rotor on the inner scale and rotor for the outer scale with a ratio of 9.7:1. The pointers will be aligned once the rotors are fully engaged. It has a serial # prefix of AF57, likely indicating manufacture date of 1957 for the US Air Force.  It was used in the H-21 Shawnee/Workhorse Piasecki Helicopter, as confirmed in its parts list dated 1957.  See photo of cockpit instrument panel illustrating this instrument. There are some markings on the glass denoting the operating limits and orientation of the glass (the white mark, just in case it rotated). Measures 3.25 inches diameter by 5 inches deep.