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it is.... In 1957, Rolex set about replacing their 1000 series of calibers. These were the first family of movements created entirely in-house by the manufacturer. It also introduced the next generation. Rather than launching the 1500 series in a great sweeping confusion across the board, they phased them in gradually over a number of years. These ran concurrently with the previous mechanisms in several models. They introduced Rolex Caliber 1520 and 1530 into this new wave. The Rolex Caliber 1530 was the first of the new wave. It was the base caliber upon which the rest of the range would be founded. The Rolex Caliber 1530 As a movement, the 1530 represented a major reworking on its predecessor. This explained Rolex’s tiptoeing approach towards its introduction. There was a host of new technology that still had to prove its worth out in the real world. Rolex originally launched it as a 17-jewel movement. Additionally, it went through a number of significant upgrades itself during its successful run before retiring in 1965. Joining the first iteration, they also released it in 25 and 26 jewel versions. Its initial butterfly rotor graduated to the familiar half-moon type. Also, they replaced the brass colored gears with the red, Teflon-coated variety we generally see today. While extremely precise and reliable, they considered the Caliber 1530 very much a workhorse caliber. Therefore very few were submitted for chronometer certification. Instead, Rolex fitted the movement into several of their non-chronometer models, such as the Air-King and the earliest versions of the Submariner, watches they initially deemed not to need mechanisms that had passed the rigorous COSC tests. However, ask any watchmaker today what in their opinion is the best movement Rolex ever produced, and many will still say the Cal. 1530. Its lack of certification was down to its relative difficulty in regulating consistently rather than its overall accuracy. Before the innovation of Microstella screws were first introduced in 1959 on the Cal. 1565, the previous screw balance was far trickier to adjust, especially considering the volume of movements Rolex produce. A comparatively low frequency caliber—18,000bph instead of the standard 28,800bph of all modern day Rolexes—the Cal. 1565 produced a five beat per second tick rather than the smoothly sweeping eight. The Rolex Caliber 1520 In 1963, in a curious and extremely un-Rolex-like move, the Cal. 1530 started its own process of being phased out. Rolex replaced it by the less advanced Cal. 1520. As the numbers suggest, it represented something of a backwards step for the usually progressive thinking company. It was centered on the same architecture as the Cal. 1530, and shared an identical base plate. However, it had several key differences, many of which were designed to keep its manufacturing costs to a minimum. Rolex only produced it with a stick regulator, rather than with the Microstella system that had found its way onto the departing 1530 by the end of its run. The Breguet overcoil of the majority of Rolex’s output was substituted for a traditional flat hairspring, and it was originally released as a 17-jewel movement, although 25 and 26-jewel versions joined the range later in the production cycle. An Impressive Performer However, even with all the cost cutting measures, the Rolex Caliber 1520 was still an impressive performer. Rolex never intended to submit it for chronometer certification. With the full weight of the company’s engineering legacy behind it, it provided the brand’s ‘Precision’ models with a beautifully built and highly accurate engine. Its increased frequency, 19,800bph up from the previous 18,000bph, gave it an extra boost in both timekeeping ability and resilience to shocks, and it remains a favorite among watch repairers for its workmanship and ease of maintenance. Along with its date function equipped counterpart, the Cal. 1525, the Rolex Caliber 1520 powered Rolex’s limited selection of non-chronometer watches until 1980. By then, all but the Air-King had gained the certification and the 3000 series of calibers had arrived, becoming the standard issue for Rolex until the present day.