Rare French 18K gold keyless chronometer Circa 1840

Gold open faced chronometer by Mathieu
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A fine and rare French 18 carat gold open faced keyless pocket chronometer, circa 1840. Gilt-brass movement, bar calibre, nine jewels, footed spring detent escapement, blued two-arm bimetallic compensation balance with gold temperature screws, blued steel flat balance spring, index regulator. Gold hinged cuvette with aperture for the folding winder. White enamel dial with Roman numerals, outer minute track, large subsidiary seconds and original blued steel Breguet hands. Polished case, coin-edge band. Cuvette signed.
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Diameter: 47 mm
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Signed: Mathieu à Paris Place de la Bourse 12