The first Cosmograph Daytona, known as the reference 6239, marked Rolex's entry into the chronograph realm when it debuted in 1963, remaining in production until approximately 1976. Offered in stainless steel and 18K yellow gold, this model boasted a groundbreaking feature: the tachymeter scale engraved on its bezel, a first for the brand. Originally marketed as the "Le Mans," it gained its enduring moniker, the "Daytona," after Rolex became a sponsor of the 24 Hours of Daytona race. The present watch is a classic example of the reference 6239 and stamped "Daytona" above the subsidiary register at 6 o'clock.

Yet, a small detail on the inside caseback, which may not be obvious to the casual observer, sets this watch apart from its peers. One would usually expect to find "6239" or even possibly "6238" punched on the inside case back of this watch. Yet, this one bears the reference 6242. A cursory search of this reference number hardly yields results. Yet, investigative research shows that these four digits are stamped on the inside case back of the one and only, reference 6262 Yacht-Master prototype.

One can thus conclude, that Rolex had originally intended to introduce reference 6242 in their line of production, only to scrap the project later. It is even possible that the reference was intended to be made for the mythical Yacht-Master line. Today, only a handful of these "prototype" watches are known on the market, one having resided in the collection of Eric Clapton. Having already produced the case backs, Rolex decided to use the existing ones in their regular line of production. The present watch was thus fortuitously fitted with a 6242 case back.