Swiss platinum lever dress watch with chronograph Circa 1928

Thin open faced chronograph with instantaneous 30 minute register
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A fine, very rare and thin Swiss platinum open faced keyless lever dress watch with chronograph and instantaneous 30 minute register, circa 1928. Rhodium plated, 23 jewels, straight-line lever escapement, cut bimetallic compensation balance, Breguet balance spring, Vacheron & Constantin micrometer regulator (Patent CH 101652, registered 1 November 1923). Matte silver dial with Arabic numerals, outer minute track with Arabic five minutes/seconds and fifth second timing scale, 30 minute register at 12, subsidiary seconds at 6 and original gold Breguet hands. Polished case. Dial, case and movement signed, case and movement numbered.
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Diameter: 46 mm
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Signed: Vacheron & Constantin Genève Suisse