Swiss ring watch with visible, diamond-set balance Circa 1820

Gold ring watch probably by Pierre Simon Gounouilhou
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A very fine and extremely rare Swiss 18 carat gold and enamel octagonal ring watch with visible, diamond-set balance, circa 1820. Brass movement, rectangular, fixed barrel, cylinder escapement, brass escape wheel, plain diamond-set balance with four spiral arms, flat balance spring. Octagonal blue champlevé enamel plate decorated with engraved foliage, in the upper part, an aperture for the rose-cut diamond-set balance spinning against a polished steel plate, in the lower part, eccentric white enamel dial with Arabic numerals and outer minute dot divisions, original blued steel spade hands. Octagonal top housing the watch, bezel decorated with gold scrolls and blue champlevé enamel, secured by two screws, engraved lower part of the band, polished sliding panel at the bottom to reveal the winding and setting holes, hinged and engraved shank with (incomplete) locking mechanism.
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Length: 30 mm
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Width: 18 mm
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Signed: No name