Swiss 18K gold keyless World Time dress watch Circa 1950

Gold lever the dial with a map of North America
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A very fine and very rare Swiss 18 carat yellow gold keyless World Time dress watch, circa 1950. Heures Universelles. The polychrome cloisonné enamel dial represents a map of North America. Polished, satiné case back and milled revolving bezel, dedication inscription on the inside. Recently serviced by Patek Philippe. Accompanied by an Extract from the Archives.
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Cloisonné enamel dial with the map of North America, gold hour indexes, outer Arabic dark and light revolving chapter ring for the day and night hours and World Time chapter ring, bearing the name of 41 locations around the world, revolving with the bezel according to the Louis Cottier World Time patent, gold hands. Rhodium-plated movement, Geneva striping decoration, 18 jewels, straight-line lever escapement, cut bimetallic balance adjusted to heat, cold, isochronism and five positions, blued steel Breguet balance spring with swan-neck micrometric index. Signed on the dial, case and movement.
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Diameter: 45 mm
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Signed: Patek Philippe & Cie Genève
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The World Time system was invented around 1935 by Louis Cottier (1894-1966), a well-known independent watchmaker in Geneva. This World Time system was used by Patek Philippe and by other major names in the Swiss watch industry, such as Agassiz, Rolex and Vacheron & Constantin. Patek Philippe created various series of World Timers from 1937 to the mid-1960s.