Very thin French/Swiss platinum knife edged watch Circa 1930

Platinum open faced keyless lever
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A very thin and rare French/Swiss platinum open faced knife edged (montre couteau) manual wind pocket watch made by Cartier in Paris, circa 1930. The rhodium plated movement with 19 jewels, a straight line lever escapement, cut bimetallic compensation balance adjusted to eight positions, blued steel flat balance spring and an index regulator. Matte silver engine turned dial with Roman hour numerals, an outer minute track with Arabic five minute numerals and blued steel decorative hands. A polished platinum case. Dial signed Cartier.
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Diameter: 49 mm
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Signed: Cartier
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Couteau is the term used by Cartier in their manufacturing records to describe the type of ultra thin watches with knife edged cases, open faced or hunting cased models. These watches, with thicknesses of less than 4 mm for the open faced and less than 5 mm for the hunting cased version, are among the thinnest mechanical watches ever produced.
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For these thin watches from the first half of the 20th century, Cartier used movements from Edmund Jaeger (Jaeger-LeCoultre), who had just invented a movement only 2 mm thick. In the 1920s, Cartier and Jaeger formed a joint venture, the European Watch & Clock Co., whose signature can be found on many Cartier movements. Movements were further sourced from Vacheron & Constantin, who, from 1940, utilized a movement which was only 1.31 mm thick.