Swiss chronometer with Guillaume-type balance Circa 1919

Gold open faced precision lever by Vacheron & Constantin
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A very fine and rare, Swiss 18 carat yellow gold open faced keyless-winding lever pocket chronometer with Guillaume-type balance, circa 1919.
Gilt brass keyless movement, 21 jewels, wolf’s tooth winding, blued steel Breguet balance spring with double overcoil, lever escapement, brass Invar Guillaume balance, index regulator with patented swan-neck spring and micrometric screw, eight adjustments, no. 386090. Plain polished round-shaped 18 carat yellow gold case. Enamel dial with Arabic radial hours, outer minutes track, subsidiary dial for the seconds, and Breguet-style hands. Dial, case and movement signed. Accompanied by the extract from the Archives, and a copy of the invention patent.
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Diameter: 45 mm
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Signed: Vacheron & Constantin, Genève Suisse
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The extract from the Archives, dated November 10, 1994, mentioned that this movement was made in 1919 and sold in 1920. The movement was completed in 1919 and delivered on April 12, 1920, to Shreve & Co., New York (U.S.A.).
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Movement based on the Swiss invention patent No. 31 052, delivered on May 24, 1904, to George Marin Grandjean, Genève (former Director of the Ancienne fabrique Vacheron et Constantin, Société Anonyme), for a “Dispositif perfectionné pour la manoeuvre de la raquette.” (Improved device for the manoeuvring of a regulator).