From 1944, this is an AN-5837-1 Altitude Correction Computer made by the Cruver Manufacturing Company.  The AN in the model number usually means Army/Navy, so I believe this is a US military aviation item.  Based on my examination of it, it appears to indicate density altitude.  Density altitude is the altitude at which, under standard atmospheric conditions, the density would be equal to the observed density.  It is used in aircraft performance evaluations.

The airfield elevation is dialed into the slide rule opposite the airfield observed pressure.  Using the second cursor, the density altitude is read opposite the observed mean dry bulb temperature.  There is no correction for humidity.

The slide rule is made of plastic and is 8 inches in diameter.

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