This is the Skytach Optical Helicopter Tachometer from Model Avionics. This improved SkyTach has a 3190 max RPM, larger viewing window and redesigned buttons.
RPM Range: 1000 – 3190rpm
Accuracy: +/- 2.5rpm
Operating Battery Drain: 29mA @ 9V
Sleep Battery Drain:70uA @ 9V
Sleep Time: 30 Seconds
Battery Type: 6LR61 Alkaline 9V battery