I acquired a small number of vintage (WWII era) aviation gauges. The others I have been able to successfully identify, but this one is a bit of an odd duck. It is marked Kollsman Type C-14, Part No. 586BK-101, Serial No. AC-42-781. I believe this places it as manufactured in 1942 and is a Kollsman Altimeter. The face of the gauge, however, is clearly marked as Gage Vacuum Pressure In. of Mercury and embossed with AF US ARMY. Could this just be an altimeter that was converted to a barometer for some reason?
This is likely a suction gauge. Specifically, this gauge must read between certain values to assure the pilot that flight instruments operated by an engine-driven suction pump are operating correctly. For instance, for the P-51D, the flight training manual states the following -
The suction gage shows whether the vacuum pump is providing proper vacuum for the system. If this gage reads more than 4.25 or less than 3.75, you know that the vacuum instruments are not functioning properly and are not giving reliable readings. Normal suction reading is 4.00.
The manual states that these instruments, namely, the flight indicator, the bank-and-turn indicator, the directional gyro, and the suction gauge, are all operated by the engine-driven suction pump. That this instrument is embossed AF US ARMY indicates that it was a part of the flight instrument group for a military aircraft possibly used during the Second World War. (Note: The government spelling of gauge is taken as gage.)
This is a Vacuum Pressure gauge that would have been used to indicate the system vacuum pressure in an airplane.