Glass hydrometer with mercury ballast, and paper label marked “Pat. Appl'd For PER CENT OF PROOF SPIRITS TP. 60° F. AMERICAN INSTRUMENT CO. WASHINGTON, D.C.” and in red ink “2631” and “1935.”
Ref: George K. Porter, “Hydrometer,” U.S. Patent 2,195,339 (March 26, 1940), assigned to American Instrument Co.
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