Tachometer, Chronometric, Type C-11, WWII, Air Corps US Army
Here is an example of the WWII-era Chronometric (i.e. mechanical) Tachometer, Type 11, as used by 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 has a serial number prefix of AF 43, evidence of a year of manufacturing of 1943, and is marked "AC USARMY" on its face. This example was made by Jaeger, part number 4770.
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-11 displayed engine RPMs in hundreds, from 0-4500.
This example is definitely experienced by in terrific condition given its age. Measures 3.25 inches diameter and ~2.5 inches deep.