Reference 6239, the very first model of the celebrated "Daytona" series, succeeded reference 6238 and could be found in sales catalogues in Europe as of 1964. It is thought though that the model was available in the US already in 1963. Available in stainless steel, 14K and 18K gold, reference 6239 was the firm's first chronograph with the tachometre scale engraved on the bezel and the subsidiary dials printed in a different color than the main dial for better readability. The dials of the early models featured only the designation "Cosmograph", referring to the outer space travels of the early 60s. The "Daytona" was added on later series to mark Rolex's sponsorship of the renowned NASCAR stock car race.