This reference 1527 Patek Philippe is a one off, oversized for the time, perpetual calendar with moonphase and chronograph. It is a solid 2.5mm larger than even the more contemporary reference 1518, which was the world's first perpetual calendar chronograph. The case of this Patek Philippe is dramatically different than the 1518. As Christies puts it "the case body has more roundness than reference 1518 and the lugs have grown in length and are more curved - smoothly clinging around its wearer's wrist. Thanks to the much more generous use of natural roundings and less of architectural lines, the watch has not only grown in size compared to reference 1518 but also gained a much more sensual, even voluptuous appearance."
What makes this timepiece even more valuable is that up until recently, reference 1527 was not known to be a complicated watch at all. In 2004, one was discovered that contained a perpetual calendar (but no chronograph), and that example belonged to the Stern family themselves, who happen to own Patek Philippe. This example, which contains both a perpetual calendar and a chronograph in a rare oversized case is truly one of a kind, and undoubtedly one of the most important finds in recent Patek Philippe history.