Rolex chronographs are grouped into two categories - Daytonas and "others". And while Rolex has been making chronographs since the 1930s, it wasn't until 1963 that the line of watches was categorized and named - and Daytona wasn't its first name. The original concept for this racing-inspired watch was The Rolex LeMans, and you can even find early advertising copy where the a true Mark I Daytona - is called LeMans. Needless to say, the name didn't stick, as Rolex's push into the American market and into official sponsorship of the 24 hour race at Daytona led to the naming of the reference 6239 Cosmograph as the Daytona. That didn't happen until 1964, and the watch in question here was produced in 1963, but we'll still call it a Daytona because it launched the reference, and thus the entire Daytona line.
But what made the 6239 so different from the 6238? it was the first time Rolex used inverse colors for sub-dials - all previous Rolex chrono dials were completely monochromatic. Also, the tachymeter scale - used for measuring distance over time (aka, speed) was moved from the dial to the bezel. May not seem like much, but it was a big deal, and these two traits made for a much more interesting, aggressive look when the 6239 hit the market. These are still traits of the Daytona today, and so the specifics that make a Mark I (one that was produced in 1963) so special.
credits Thanks to H
