French 18K gold knife edged keyless pocket watch Circa 1920

Gold Art Deco watch 
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A French 18 carat gold open faced knife edged (montre couteau) keyless pocket watch made by Cartier in Paris, circa 1920. Extra thin, Art Deco dress watch with a polished case back. Gold engine turned dial with Arabic hour numerals, an outer minute track, subsidiary seconds at 6 o’clock and blued Breguet hands. A gilded brass movement, straight line lever escapement, going barrel, eight adjustments, cut compensated balance and blued steel hairspring with terminal curve, polished steel index regulator. The dial, case and movement are signed.
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Diameter: 48 mm
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Signed: Cartier Paris
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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.