The rarity of this particular watch consists in the date and moon auxiliary dial which is not encircled and marks the transition between Ref. 1518 and the later Ref. 2497 and Ref. 2499. This particularity renders this watch a rarity and worthy of the most demandings collector.Ref. 1518Produced in 281 examples from 1941 to 1954, in yellow and pink gold, and only three examples known to date in stainless steel.- First series with applied gold Arabic numerals from 194 to 1954.- Second series presented at the Basel Fair in 1942 with enameled hour indexes, the 12 in applied gold Arabic numerals.- Third series with applied gold indexes, the 12 in applied gold Arabic numerals.- Fourth series with applied gold "Bâton" indexes.Similar watches are published in "Patek Philippe Wristwatches", 1998 Edition, by Martin Huber and Alan Banbery, p. 303 and in "Collecting Patek Philippe Watches" by M. and O. Patrizzi, Guido Mondani Editore, Genova, 2000, pp. 21-47-292-293-294 and 295.What is a Perpetual Calendar?Mechanical movements tell time by means of ratcheted wheels turning in synchronization. The number of teeth on each wheel describes its function. There are separate wheels for the seconds, the minutes and the hours.A perpetual calendar movement has additional wheels, levers and springs. It has wheels for the days of the week, the day of the month, the year, usually one for the phases of the moon and usually one that makes just one revolution every four years, synchronized with all the others, to account for February 29th in the leap year. Audemars Piguet said they produced the first wristwatches with the perpetual calendar and moon phases in 1924. In 1925, Patek Philippe modified a lady's pendant watch witperpetual calendar to produce the world's first instantaneous changing perpetual calendar wristwatch with indication forleap years.