Tachometer, Chronometric, Type C-7, WWII, Air Corps US Army
Here is an example of the WWII-era Chronometric (i.e. mechanical) Tachometer, Type 7, as used by early single engine aircraft of the Air Corps US Army. According to the US Army Air Force Maintenance Interchangeability Chart dated Feb 1945, the C-11 tachometer was used in fighters P-40 Warhawk (E-N), early versions of the P-47 Thunderbolt, and P-51 Mustang, dive bombers A-35 Vengeance and A-36 Apache, and transports UC-61 and UC-64A. This tachometer is marked "AC USARMY" on its face, for Air Corps US Army. This example was made by Jaeger, part number 2300.
The chronometric tachometer was connected mechanically and directly to the engine via flexible cable shaft, which was eventually replaced in later aircraft with electrical tachometers. The C-7 displayed engine RPMs in hundreds, from 0-2500 in one rotation of the pointer, and then from 2500-3500 as the pointer continues past the 12 o'clock position and read along the inner scale to the 4 o'clock position. The C-7 tachometer was illuminated by an intergral lamp located in the upper right corner. The lamp would be lit by twisting its knob. The lamp is in place by unlikely working. On the rear, the electrical connector is protected by a removable metal cap.
This example is definitely experienced but in terrific condition given its age. There is slight haze on the lower inner glass but could probably be opened and cleaned. Measures 3.25 inches diameter and ~2.5 inches deep.