Tachometer, Chronometric, Type C-9A, WWII
Here is an exceptional example of the WWII-era Chronometric (i.e. mechanical) Tachometer, Type C-9A, 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-9A 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 AC 41, evidence of a year of manufacturing of 1941, and is marked "AC USARMY" on its face. This example was made by Jaeger, part number 3800.
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-9A displayed engine RPMs in hundreds, from 0-2500 in the first pass of the pointer along the outer scale, and then from 2500 to 3500 RPM along the inner scale.
This example is definitely experienced by in very good condition given its age. The operating limits of the last aircraft in which it was installed are painted on the face of the aircraft. Measures 3.25 inches diameter and ~2.75 inches deep.