References 6050 and 5050 are identical with the exception of the 6050 having a ?Super-Oyster? crown. The Ref. 5050 (See lot 176) has a traditional Oyster crown. Reference 6050 was probably the last of the so-called ?Bubble-Back? models. This type of case is called ?Bubble-Back? by Englishspeaking collectors. The Italian collectors call the watch an ?olivetto? due to the olive like outline of the case. 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. The calibre 9 3/4??? Ref. 620 was produced between 1940 and 1951. This watch is illustrated in ?Rolex Collecting Wristwatches? by Osvaldo Patrizzi 2001 Edition p. 155.