Reference 2509 was launched in the early 1950's with the caliber 12-400, and is distinguished by its water-resistant case and screw back. Stamped with the case-maker mark “FB” for Francois Borgel (later acquired by Taubert & Fils), the patented case back uses a distinct ten-sided design on the case back for easier gripping when opening or closing the back. While the reference 2508 was Patek Philippe’s first use of this waterproof case on a non-complicated watch, the 2509 followed soon after.

The 2508 used the caliber 27 SC, with sweep center seconds, while the 2509 contained the caliber 12-400, with subsidiary seconds. With the majority of the reference 2509’s being cased in yellow gold, examples in stainless steel or pink gold (such as the present watch) are a rarity. Preserved in excellent original condition with a crisp hallmark below the crown and sharp case facets, the 2509 epitomizes an elegant Patek.