ROLEX introduced reference 5512 in 1959 to substitute for Ref. 5510 and 6538, starting with a 478.xxx serial number. The most important features of the new reference were a new wide graduated revolving bezel for the decompression times, a smaller winding crown (from 8 to 7 mm.) and a protected winding crown called the "crown guard". The earliest models featured square type guards; approximately 50 examples of Ref. 5512 were tested by professional divers. It came out that this type of crown guard was not easy to use while wearing professional diver's gloves and Rolex stopped production. To the best of our knowledge, a few dozen Ref. 5512 with square guards were made, starting with serial number 477.9xx, and only in 1959. Only a few examples have appeared on the market. Due to its rarity this model is much sought-after by Rolex collectors.