The Patek Philippe ref. 2523 HU (Heures Universelles), one of Patek’s greatest ever references and one that will forever be in the sights of the world's leading collectors. The term “grail” doesn’t even come close. Hopefully I can give you some idea as to why that is.
Released in 1953 and produced for just 4 short years up until 1957.
However to best understand the ref. 2523 we need to go back to 1931 when a gentleman by the name of Louis Cottier (1894-1966), a watchmaker from Geneva, invented the Heure Universelle (world time) system. He would then spend the next few years approaching the likes of Rolex, Vacheron Constantin, and of course Patek Philippe, trying to get them to use his world time design in their references, all of which eventually did. It wasn’t until 1937 that we would see the first Patek Philippe world time pieces start to be produced.
The first of which is the thought to be unique, the ref. 515 HU in pink gold (I have also heard of possibly 3 more ref. 515 prototypes), a rectangular Art Deco cased gentleman's wristwatch with fixed internal bezel featuring the cities of the world.
In 1937 we also saw the release of 3 other world time references, the ref. 96 HU of which 2 pieces are known, and the ref. 542 HU of which just 5 examples are thought to have been made and saw the introduction of the rotating bezel featuring 31 cities from around the world and also the first Patek Philippe to indicate noon and midnight using a sun and moon in the 24 hour ring.
The third reference isn’t in the shape of a wristwatch but a pocket watch, the ref. 605 HU. This reference was produced for almost 30 years from 1937 all the way up until 1964 with approx 95 pieces made in total. Due to its long life span the ref. 605 HU was produced alongside the next world time reference and the first to be serially produced, the ref. 1415. Produced between 1939 and 1954 with approximately 115 pieces made this reference really put Patek Philippe and Cottiers world time design on the map! (pardon the pun). The importance of the ref. 1415 could still be seen and appreciated many years later when in 2002 the only known platinum example appeared at auction and achieved a then world record for a wristwatch of 6.6 million Swiss Francs!