Tachometer, Sensitive, Type E-30, Kollsman
This is a Sensitive Tachometer, Type E-30, made by Kollsman, part number 2185-4-03, for an order by the US Air Force with a serial # prefix indicating year of manufacture of 1954.
The dial markings are clear and the glass and case are intact. Measures 2 inches diameter by ~5 inches deep. See photos.