This is an interesting slide rule produced by the Refrigeration Service Engineers Society (RSES) in 1964.  While not designed for meteorological use, it is interesting because it has scales for the latent heat of air, the vapor pressure, and the specific humidity in various units, presumably at 30 inches pressure. It uses the wet bulb and dry bulb temperatures as input.
The calculator is about 8 inches across.  It's printed on heavy card stock.  The reverse side is blank.

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