Fine French 18K gold open faced lever chronometer Circa 1870

With fusee, unusual winding and hand-setting to improve precision 
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A very fine and rare French 18 carat gold lever chronometer pocket watch with fusee, unusual winding and hand-setting to improve precision, circa 1870. Special calibre, matte gilt movement, fuse and chain with Harrison’s maintaining power, counterpoised straight-line lever escapement, cut bimetallic compensation balance with gold meantime and temperature compensation screws, blued steel helical balance spring, the terminal secured by a stud with two screws, polished steel rack and pinion regulator. Gold hinged cuvette with an aperture for regulation. White enamel dial with Roman numerals, outer minute track and Arabic five-minute numerals, subsidiary seconds and original blued steel spade hands. Polished case, the back with engraved monogram, button at 1 for engaging the winding, at 11 for hand-setting. Cuvette signed.
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Diameter: 50 mm
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Signed: Gustave Sandoz Horloger de la Marine Nat.le Paris
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This watch uses fusee and chain power. Only precision watches such as chronometers were made with fusee by this date. This is because the fusee gives a more stable and consistent supply of power as it runs down, also the maintaining power provides power to the escapement during the winding process, again ensuring no loss of timekeeping. The winding is only engaged when the button at 1 o’clock is depressed.
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Gustave Sandoz Geneva and Paris. The firm apparently began in Paris and was listed there in 1867, 1870 and 1900. It won Second Prize and Honorable Mention at the Geneva Chronometer Contest in 1882.