This is a US Navy ML-455 Meteorological Conversion Scale. It is made of plastic and was manufactured by the Virginia Plak Company, which was founded in 1970.

It's not a slide rule in that it doesn't have a moving slide. The front side has a temperature conversion scale from Fahrenheit to Celsius (or vice versa). It also has a pressure conversion scale from millibars to inches of mercury (or vice versa). The reverse side is covered by a gold sticker that's peeling off. It has conversion scales between calories and watts and between degrees Celsius and degrees Kelvin, as well as several constants useful in meteorology.

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