This is a US Army Signal Corps ML-323 Pressure Calculator from 1945.  It was made by the Virginia Plak Company.  Undoubtedly for aircraft use, the Air Force was still part of the US Army in 1945,  The airfield sea level pressure in inches of mercury is dialed in on the slide rule.  The user can then read the pressure in millibars as a function of altitude, or vice-versa.  The front of the slide rule is used for altitudes of -1,000 to 23,000 feet, while the reverse side is used from 21,000 to 45,000 feet.  The airfield pressure needs to be reset when switching from the front to the reverse sides. There is no adjustment for air temperature or humidity.

The slide rule is made of plastic and is 6 inches in diameter.

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