Patek Philippe, model: Ref.725-2, open-face perpetual calendar moon phase pocket watch, produced in the 1950s, 18K gold, French day dial. Four-sub-dial layout: upper left week dial (SAM / DIM / LUN / MER / JEU / VEN), upper right month + leap year display, lower right date, lower left moon phase + small seconds hand. The dial is in its original warm and smooth state. It features three-dimensional rose gold Arabic hour markers and hands, and a classic four-sub-dial perpetual calendar structure. This Ref.725-2 belongs to the four-sub-dial version of the first major series of Ref.725 and is also the most classic generation of this series. Launched in 1940 and produced in the 1940s and 1950s, the four sub-dials independently display the day of the week, month, leap year, date, moon phase and small seconds. The subsequent Ref.725 / 4 is the second series, featuring an American linear calendar window. Ref.725 was PP's first standardized mass-produced perpetual calendar pocket watch, laying the design foundation for subsequent perpetual calendar wristwatches.