A similar watch is published in Patek Philippe Wristwatches, by Martin Huber & Alan Banbery, 1998 Edition, pp. 243-245.The World Time system was invented around 1935 by Louis Cottier (1894 - 1966), famous independent watchmaker in Geneva. This World Time system was used by Patek Philippe and also by other major names in the Swiss watch industry, such as Agassiz, Rolex and Vacheron Constantin.Patek Philippe created various series of ?World Timers? and the earliest known is a probably unique rectangular ?galbé? pink gold model, Ref. 515, made in 1937, which was soldby Antiquorum, Geneva, April 10, 1994, lot 502, forSFr. 550'000 (US$ 388,560).From 1937 until about 1940, Patek Philippe used the ?World Time? movement in various types of cases:Ref. 542 (Ø 28 mm.), of which only 4 examples are known to date and three were sold by Antiquorum:- Geneva October 17, 1993, lot 431, at SFr. 121?000(US$ 81,700).- Geneva, October 15-16, 1994, lot 608, at SFr. 110'000(US$ 87,300).- Geneva, April 10, 1994, lot 500, at SFr. 231'000(US$ 164,200) with unusual bezel and hands.Ref. 96 Calatrava, which appears to be an unique example, made in 1939.Around 1939-1940 began the production of the seriesRef. 1415 (drop-shaped lugs), together with three examples of the Ref. 1416 (straight ?claw? lugs), both references with a diameter of 31 mm., and, on the revolving bezel the names of 30 cities of the world.A few examples of the Ref. 1415 were then made witha revolving beel featuring 31 cities of the world.In 1940, Patek Philippe created, specially forDr. P. Schmidt, a unique ?World Time? chronograph(Ref. 1415-1) wristwatch, in yellow gold, with square push-buttons and the names of 33 cities of the world, which was auctioned by Antiquorum, Geneva, October 15-16, 1994, lot 516, setting a World Record at SFr. 990'000(US$ 785,714). This watch is illustrated on the front cover and p. 270 of the new edition (1998) of Patek Philippe Wristwatches, by Martin Huber & Alan Banbery.From the mid-1940, Patek Philippe produced ?World Timers? in three main series which include dress watches (Ref. 605, circa 1940-1960?s), some with a matt silvered dial or black dial and others with a cloisonné enamel dial; and, wristwatches, with one crown (Ref. 1415, circa 1939-1950?s), and two crowns (Ref. 2523 and Ref. 2523-1, circa 1953-1965), some with a matt silvered or gilt dial, some with a guilloché dial, some with an enamel dial and some witha cloisonné enamel dial. The revolving bezel of theRef. 1415 features the name of 41 cities of the world, and the revolving ring dial of the Ref. 2523, the name of40 cities of the world.World Time Patek Philippe wristwatches are published in Patek Philippe Wristwatches, by Martin Huber & Alan Banbery, 1998 Edition, pp. 240 to 247.