Comex's collaboration with Rolex started in early 1971. Prior to that date, Comex had cooperated with Omega, for the testing of the design of the Ploprof (short for plongeur professionel), of which a prototype series was tested during Janus II and Physalie V (Nov. 1970) However the Ploprof had a major problem, with its ?explosion? during the decompression time after a saturation dive, due to the helium build-up inside the case. Rolex had noted this problem on the early Sea-Dwellers that were not fitted with helium valves and quickly moved to solve this problem by fitting the valves on the side of the case (patent number 492246). As the diving development continued, Rolex continue to collaborate and test their watches with the use of professional divers, now with Submariners, which were largely sufficient for dives up to 200 meters. During testing of the 5513 they experienced similar problems with crystals exploding upon decompression. So Rolex equipped Ref. 5513 with a valve and a very limited series of Submariners was delivered to Comex Saturation Divers for testing (approximately 20 pieces). This particular batch of watches could be considered the true pre-production prototypes of the 5514, as they were designed to be tested in real-life conditions and bore the following characteristics: they did not feature the Comex logo on the dial (this was to come on the later production model 5514) and only had the Comex engraving on the case back and a small progressive number.