Notes: When the first series of ref. 6538 Submariners was issued to the divers of the Royal Marines’ Special Boat Squadron (later to become the Special Boat Service), the divers noted one problem with the watch — it was difficult to rotate the bezel when wearing diving gloves. The Royal Navy also wanted the watch to have solid strap bars so Rolex decided to make a special model just for the SBS, initially carrying the unique reference 6540. But as the quantity needed was so small — the initial order was for only 21 watches — it was decided to make these watches as a spin-off from the ref. 6538, and they were called the A/6538.

Mechanism: The Oyster Perpetual began life with the calibre 1030, in 1950. This calibre was also used in several Submariner and Explorer references, 65xx and 66xx. The Cal.1030 was the most popular 18,000 bph movement at the time it was being made. It was noted for its use of a mono-metallic compensation balance and its blue steel Breguet overcoil hairspring.

Cal.1030 also features an unique rotor which is totally different with today’s, two prongs are hollowed on semi-circle, around the outer race engraved with word of “ROLEX PERPETUAL”, beneath is the word of “PATENTED” and the iconic patter of the Rolex crown.

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