French gold watch set in US 20 dollar gold coin dated 1907 Circa 1920

Gold lever coin watch by Cartier
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A very fine and extremely rare French, 18K yellow gold U.S. 20 Dollar coin dated 1907 set with a concealed keyless watch, circa 1920. Double case, coin opening with a push-piece concealed in the band, a two-body hinged inner case with two screws, Matte silver dial with painted radial Roman numerals, outer minute track. Blued steel Breguet hands. A rhodium-plated movement, Geneva striping decoration, 18 jewels, straight-line lever escapement, cut bimetallic compensation balance with eight adjustments, blued steel flat balance spring, and an index regulator. Dial and movement signed Cartier.
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Diameter: 34,5 mm
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Signed: Cartier  Paris
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The dial is slightly oxidized.