Tachometer, Helicopter, USSR 0-4000 RPM
Here is a Cold War-era electronic dual Tachometer, 0-4000 RPM, as used in military helicopters of the USSR and Poland such as the MIL MI-1 (see cockpit photo, the tachometer is in the middle of the second column of instruments from the right) and MIL MI-4 (see cockpit photo, the tach is in the bottom row 3rd instrument from left).
The pointer labeled "P" indicates RPMs of the Rotor, while the pointer labeled "M" indicates RPMs of the Engine. It is in exceptional condition for its age. Measures 3.25 inches diameter and ~5 inches deep.