The reference 2509 introduced Patek Philippe's earliest water resistant case. The water resistance problem was addressed via the massive screwback, which was a huge functional improvement over the ordinary snap-on caseback.
Case is 18k rose gold with dimensions of 35mm by 9mm thick. Generously domed acrylic crystal. Oversized winding crown.
Movement is the Patek Philippe manual winding Caliber 27SC with 18 jewels, hand adjusted and stamped with the Geneva Seal.
Dial is silver with applied golden hour markers and dauphine hands. Subsidiary seconds dial at 6 o'clock.
This watch is in overall excellent condition and is unpolished. It is heavier than other Calatravas of its period due to the massive screwback. This is a classic and important Patek Philippe wristwatch.