Rolex first introduced the Submariner in the early 1950's. It is probable that the Submariners original design was loosely based on the Rolex Explorer. However, the Submariner was made with a case that was much thicker than the Explorer, with a rounded crystal and oversized crown. The early Submariner models had stick hands with luminous tipped second sweep hands. When this was first introduced, the Submariner was water resistant to 330 feet (100m). The first Submariners also were designed with a bi-directional bezel without the minute hash marks (as is commonly seen today). Initially, the Submariner was fitted with the A296 movement but after a few years, Rolex upgraded the movement to the 1030, added the Mercedes hands and gave the models a new model number 6536/6538. Rolex also included round luminous hour markers and rectangular luminous markers at 3, 6 and 9 positions. The late 1950s also saw the introduction of the "shoulder" feature (where the sides of the case come up on either side of the crown and create a protective shield around the crown). The original 6200 was renumbered as well and the movement upgraded to the 1530 non- chronometer version. By the early 60's all the movements in the Submariner range were upgraded to the Chronometer version. In the mid-60's Rolex gave the submariner yet more features and upgrades with a new movement and added the date feature with the cyclops bubble. During the 50’s and 60’s Rolex were contracted to provide the Submariner to the Royal Navy. Unlike the consumer models, the Royal Navy watches had welded steel bars instead of the spring bars. Also, these watch hands were wider and bolder with a simplified crown. These Royal navy edition Submariner watches also had engravings on the back for easy identification. Rolex kept fine-tuning the Submariner by adding the triple-lock crown in the 70's and the scratch-resistant synthetic sapphire crystal in the 80s. The introduction of the sapphire crystal also helped Rolex streamline the profile of the Submariner. The 80s also saw a renumbering of the model numbers from the 1680 to the 16800. This was done primarily to help in easy identification of Rolex model variations (stainless steel, two-tone, 18K gold, etc.). In addition, the operating depth of the watch was also increased to 1,000 feet. In the mid-90s, Rolex again updated the model to the 16610 with the 3135 caliber movement. Later in the 90's a new movement was included without a change in the model number. The final possibly most major change was the introduction of the ladies Submariner in the mid-90s. In 2003, marking the 50th anniversary of the Submariner, Rolex introduced the maxi dial, green bezel Submariner.