This type of case is called ?Bubble-Back? by English speaking collectors. The Italian collectors call the watch an ?ovetto? due to the olive like outline of the case. Ref. 6015 and 5015 are identical except that the 6015 has a ?Super-Oyster? crown and the 5015 (see lot 175) has a traditional Oyster crown. The caliber 620 was created in 1936 and was in regular production between 1940 and 1951. The ?Super-Oyster? winding crown was an attempt by Rolex to create a waterproof winding crown which did not need to be screwed down. The system itself worked however in time the rubber seal perished and this crown type was phased out after only a few years of production. This watch is illustrated in ?Rolex Collecting Wristwatches? by Osvaldo Patrizzi 2001 Edition p 152.