The inside of the caseback bears the stamp 'FB Patent', which is the hallmark for F. Borgel, a case maker who worked in Geneva for Patek Philippe and other watch companies. Borgel's specialty and patent was for water-resistant screw-back cases, such as the present example.
In the 1950s it was common for Patek Philippe to work in tandem with case making companies to produce high quality, specialty cases. Examples in 18K pink gold, such as the present lot, are rather rare.
An archival image of this model, reference 2509, is illustrated in Collecting Patek Philippe Watches, by Madeleine & Osvaldo Patrizzi, p. 52. It is described in Patek Philippe catalogues from the 1950s as being impermeable, or waterproof.