Extremely rare Swiss 18K gold precision pocket watch Circa 1910

Gold lever with open face by Vacheron & Constantin
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An extremely rare Swiss gold open faced keyless wind pocket watch with special calibre, an anchor chronometer, with 21 stones, circa 1910, made for Caldwell & Co. USA.
Polished 18K gold case with engraved monogram, case number 291462, and factory number 346892. Signed enamel dial in very good condition with Breguet-style radial hours, outer minutes track, subsidiary dial for the seconds, and blued steel Breguet-style hands. Extremely rare calibre with 21 jewels, screwed gold chatons, fine adjustment, special anchor.
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Diameter: 47 mm
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Weight: 80 g
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Signed: J.E. Caldwell & Co.