Helicopter Dual Tachometer Kollsman 981BX-8-023 H-21 Piasecki F-104 it would seem to be

it would seem to be for a dive bomber

Reference Number 41-7952

It is in superb condition given its age

Magnesyn Type C-25

One has a service date of 1973 stamped on its side

Helicopter Dual Tachometer Kollsman 981BX-8-023 H-21 Piasecki F-104 it would seem to beHere 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