French/Swiss 18K yellow gold knife edged pocket watch Circa 1925

Gold keyless lever dress watch
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A very fine and rare, thin knife edged (montre couteau) keyless dress watch made by Cartier in Paris, Circa 1925. The polished open faced case, crafted from 18 carat yellow gold, has a triangular bow and its back a central blue champlevé enamel monogram. The matte silver dial with engine turning, Roman hour numerals, an outer minute track with Arabic five minute numerals and blued steel halberd hands. The rhodium plated movement has Geneva striping decoration, 19 jewels, a straight line lever escapement, cut bimetallic compensation balance with eight adjustments, blued steel Breguet balance spring and an index regulator.
Dial signed Cartier, case numbered and stamped EJ in a lozenge for Edmond Jaeger, movement signed European Watch & Clock Co. Inc. Swiss.
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Diameter: 47 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.