Reference 6239 was introduced in 1963, which was a breakthrough year for Rolex. Named the "Cosmograph”, the model featured for the very first time a tachymeter scale on the bezel. While originally announced as the "Le Mans" it was eventually called "Daytona”.

This watch is among the earliest reference 6239s ever produced and displays all the correct attributes of the popular "Double Swiss Underline". The Mk 1 bezel is calibrated to 300 units per hour and features small hash marks. It is also recognizable by the 275 intermediary unit, which cannot be found on later examples. The black "Double Swiss Underline" dial is most compelling. Rolex had originally used dial stamps that were designed for use on previous models. Since the bezel of the new "Cosmograph" was larger and covered the original printing, Rolex redesigned the stamps and printed "Swiss" the second time. Once the dial was fitted, the inscription was immediately visible again.

Most striking is the underline below the Rolex Cosmograph signature, which scholars believe was used to signify a transition from radium to tritium on the dial. Other small details pull the watch together, such as the 6238 case back, which is correct as Rolex usually used the leftover case backs from previous models. The movement is stamped ROW, indicating it was made for the American market – another typical attribute. The hands are also slightly longer and thinner, almost touching the edge of the hour markers. This watch appeals to purists “in the know” who seek a subtle yet interesting Cosmograph.