Rare Patek Philippe model will be exhibited at Christie's auction in Geneva next month. Chronograph in 18 carat gold with perpetual calendar and moon phases in 1943 is estimated at 1, 5 - 2, 5 million. Swiss francs ($ 1, 4 - $ 2, 4 million.) Watch Patek Philippe ref. 1527 first appeared at auction 20 years ago and has since become one of the most coveted trophies for collectors. This chronograph became the ancestor of the next generations of Patek Philippe. Model 1527 exists in a single copy, which, combined with historical value makes it one of the most expensive on the planet. In total, the auction Important Watches, to be held on May 10 will be sold 90 different models of Patek Philippe.