Helicopter Dual Tachometer Kollsman 981BX-8-023 H-21 Piasecki F-104 Magnesyn Type C-25

Magnesyn Type C-25

it would seem to be for a dive bomber

but also on the US Army Air Corps' UC-43 Beech Staggerwing (see photo from flight manual dated 1944)

Type 99 Trainer

Helicopter Dual Tachometer Kollsman 981BX-8-023 H-21 Piasecki F-104 Magnesyn Type C-25Here 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