Ref. 1526Produced in 210 examples from 1941 to 1952, with the calibre 12'''-120 QP, the majority in yellow gold, rarely in pink gold, with only one example known to date in stainless steel.- First series with indirect centre-seconds produced in very few examples. One of these watches was sold by Antiquorum, Geneva, in the theme auction The Art of Patek Philippe, April 9, 1989, lot 203.- Second series with concentric auxiliary dials for the seconds and the days of the month. A similar watch is published in Patek Philippe Wristwatches, by Martin Huber & Alan Banbery, 1998 Edition, pp. 282-283.Patek Philippe "World Timers"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 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.Ref. 96 Calatrava, which appears to be an unique example, made in 1939.Around 1939-1940 began the production of the series Ref. 1415 (drop-shaped lugs), together with three examples of theRef. 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 of33 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 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."World Time" Patek Philippe wristwatches are published in Patek Philippe Wristwatches, by Martin Huber & Alan Banbery, 1998 Edition, pp. 240 to 247.