Very thin French 18K gold open faced lever watch Circa 1930

Gold knife edged pocket 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 1930. The thinnest timepiece made by Cartier, only 4 mm thick. A plain case with the French gold punch. Original, gilt engine turned dial with Arabic hour numerals, outer minute divisions and  original blued steel Breguet hands. The extremely flat, prime quality lever movement is only 2 mm thick. Dial signed (the signature partially faced).
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Diameter: 46 mm
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Thickness: 4 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.