Rare English water and dustproof deck watch Circa 1925

Silver open faced deck watch by Charles Frodsham
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A rare, water and dustproof expedition English deck watch, circa 1925.
Silver polished case with screw back, winding crown underneath a screwed protective cap and secured by a chain, and case maker’s punch mark RN (Robert Benson North, proprietor of North & Sons 1917-1933, successors to Nicole, Nielsen & Company). Signed and numbered enamel dial, off-white (Willis), with radial Roman numerals, auxiliary seconds, and blued spade hands. Frosted, gilt and signed 2/3 plate movement with screwed chatons, English lever escapement, gold screw compensation balance, and set diamond endstone on balance.
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Diameter: 58 mm
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Weight: 183 g
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Signed: Charles Frodsham By Appointment to the King, 24 South Molton Street, London.