The Patek Philippe Perpetual Calendar Ref. 725 is the pocket watch that influenced nearly every perpetual calendar the brand made over the following century, including the 1518, 2499, 3970, 3448, 5970, 5270. While many of the reference 725 pocket watches feature the four subdial perpetual calendar dial, this example features a straight line calendar with the month, date and day displayed digitally and the moonphase at 6 oclock. It is believed that Patek Philippe made fewer than 100 reference 725 watches across all metals and variations, making this a very rare bird. This example comes with an archive paper noting the original production year of 1951 and sale date in December of 1957.