Split-second chronograph wristwatches were the must of Patek Philippe technology and were generally cased after 1938 with the Ref. 1436.Ref. 1436Produced from 1938 to 1971. The majority are in yellow gold, rarely in pink gold, and three examples only are known in stainless steel, of which two were sold by Antiquorum:- Geneva, April 10, 1994, lot 431, at SFr. 484?000 (US$ 342,000)- Geneva, April 23, 1995, lot 457, at SFr. 379?500 (US$ 334,650)Some were produced with a coaxial button on the winding-crown for the split-second functions (stop and reunite).A similar watch is published in Patek Philippe Wristwatches, by Martin Huber & Alan Banbery, 1998 Edition, pp. 273-274.A probably unique, extra-large size, Ref. 2512, was made in 1950 and sold on 14 July 1952. It was offered by Antiquorum in Geneva, October 12-13, 1996, lot 644, and realized SFr. 792?000(US$ 628,600).Split-second chronographs with a waterproof case and round push buttons are extremely rare. They have the Ref. 1563 and were produced in the 1940?s. Only three examples in yellow gold are known to exist today. One of these, originally purchased in 1948 by Duke Ellington, was sold by Antiquorum in New York on June 20, 1998, lot 135, for US $ 673,500 (SFr. 1?015?000).Another waterproof split-second chronograph was cased in the 1980?s in yellow gold with round push buttons in a case of areference 2499.Poinçons de GenèveThe voluntary quality control of watches at the Geneva Observatory, was established by a law dated November 6, 1886. Conditions were laid down for the attribution and stamping of the Poinçons de Genève, punch-marks designed as the coat-of-arms of Geneva. Pocket watches, or wristwatches which carry the Poinçons de Genève, considered as an equivalent to a Bulletin Officiel de Marche, punched on a bridge and on the main plate of the movement, were qualified to be officially termed ?chronometers?.