Swiss gold chronograph with pulsometric scale Circa 1920

Gold open faced keyless lever
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A Swiss 18 carat yellow gold open faced chronograph keyless pocket watch, made by Vacheron & Constantin in Geneva, circa 1920.  Chronograph, 1/5 second, activated by the single round push piece located on the winding crown) with a pulsometric scale graduated for 30 pulsations. A polished case back, white enamel dial with Roman numerals, pulsometric scale, subsidiary at 6 and gold hands.  Movement with a lever escapement, index regulator with swanneck spring and micrometric screw. Signature on the dial, case and movement.
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Diameter: 48 mm
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Signed: Vacheron & Constantin, Geneva