Mechanism: 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.

Family Notes: The Rolex Oyster Perpetual Submariner is a line of sports watches designed for diving manufactured by Rolex, known for their resistance to water and corrosion. The first Submariner was introduced to the public in 1954 at the Basel Watch Fair. The Rolex Submariner is considered "a classic among wristwatches", manufactured by one of the most widely recognized luxury brands in the world, and due to its popularity has become the subject of massive counterfeiting. The Rolex Submariner is part of Rolex's Oyster Perpetual line. See a copy of the 1957 Submariner magazine advertisement.

The Submariner model went into production in 1953 and was showcased at the Basel Watch Fair in 1954. The assigned case reference number of this first Submariner was either 6204 or 6205. It is unclear which model came first; in any event, the two watches are nearly identical. Neither has the distinctive "cathedral" or "Mercedes" hands now so strongly associated with the Submariner line. Rather, both of these early submariners have straight "pencil" style hands. Few, if any, of the 6205 watches bear the name "Submariner" on the dial, a major distinction of modern Submariners. Some 6204 models have the Submariner logo printed below the center pinion, while others have the logo blacked out. It is believed that there were unexpected trademark issues connected with the name "Submariner" at the time the 6204 and 6205 were released, accounting for the inconsistent use of the Submariner mark on these early Submariners. Trademark irregularities notwithstanding, both the 6204 and 6205 are designated Submariner models in Rolex product literature.

From Wikipedia