This watch is particularly rare and its rarity consists in the flat bezel and the case which is shaped like a coin.A similar watch is published in "Patek Philippe Wristwatches", by Martin Huber and Alan Banbery, 1998 Edition,pp. 243 and 245.What is a "World Time" ?A "World Time" wristwatch displays the names of the major cities throughout the world. The watch contains a24 hour ring on the dial, which can be set to show the time of any of the world cities.The World Time systemwas 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 rectangular "galbé" pink gold model, Ref. 515, made in 1937, which was sold by Antiquorum, Geneva, April 10, 1994, lot 502, for SFr. 550'000 (US$ 388,560).From 1937 until about 1940, Patek Philippe used the "World Time" movement in various types of cases: Ref. 542 (O 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.Around 1939-1940 began the production of the series Ref. 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.In 1940, Patek Philippe created, specially for Dr. 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 and Alan Banbery.From 1940 onwards, 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 with a cloisonné enamel dial. On the revolving bezel of the Ref. 1415; the name of 41 cities of the world. On the revolving ring dial of the Ref. 2523; te name of 40 cities of the world.Patek Philippe World Time wristwatches are published in "Patek Philippe Wristwatches", by Martin Huber and Alan Banbery, 1998 Edition, pp. 240 to 247.