Rare English silver open faced chronometer watch Circa 1864

Rare silver open faced watch with helical hairspring
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An English rare silver open faced watch with helical hairspring, made in 1864, full plate gilt metal movement, detent escapement, helical balance spring, S balance, engraved balance cock.
White enamel dial, radial Roman numerals, subsidiary seconds dial, silver case, hinged back, dial and movement signed Arnold, case hallmarked for London 1864, makers mark JO.
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Diameter: 56 mm
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Signed: John Roger Arnold
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John Roger Arnold (1769-1843) apprenticed under his father, John Arnold, and Abraham Louis Breguet. Along with his father he founded Arnold & Son in 1787 and they worked together until his father’s death in 1799. The present lot contains an Arnold invented Z balance, a similar example of which can be found in Vaudrey Mercer, John Arnold & Son, plate 112.