Tachometer, Electric, Kollsman 725SD-01-3570
This is a sensitive electrical Tachometer made by Kollsman part number 725SD-01-3570. This tachometer has two pointers; the long pointer indicates RPMs in 100's along the outer scale, and the short pointer indicates RPMs in 1000's along the same scale. There is a thin yellow line on the face at 4,000 RPMs, and a thin red line at 7,000 RPMs. This was a lighted tachometer, with a light receptable in the upper right corner, but the lamp itself is not installed.
We have not come across this tachometer in any of our research, but assess it to be early WWII or pre-WWII era, as most internally-illuminated instruments of this type gave way to luminous dials early in the war years. It may have also been used in civilian aircraft.
Measures 3.25 inches diameter and ~5 inches long. It is in very good condition given its age, the glass and case are intact and the lettering is precise.