With a 39mm case, this vintage Royal Oak Perpetual Calendar sits much better on the wrist than the newer 41mms, with not only better proportions but also being more faithful to the original. In fact, the movement inside this watch is the same you will find on the Jumbo models as it is the legendary JLC Cal. 2120 with a perpetual calendar module built on top. As a result, this watch shares the same thickness as the 15202 with a slightly thicker bezel to accommodate the perpetual calendar module. For such a complicated watch that takes into account how many days each month has, including leap years, this is truly impressive. While the current Royal Oak Perpetual Calendar is fairly common, the 25820, especially with a smooth grey dial, is extremely rare. Just try looking around for another, you won’t find too many as they were made in low numbers.