ROLEX | COSMOGRAPH DAYTONA, REFERENCE 6240 A STAINLESS STEEL CHRONOGRAPH WRISTWATCH WITH BRACELET, CIRCA 1968" | 勞力士 | Cosmograph Daytona 型號6240 精鋼計時鏈帶腕錶，錶殼編號1658525，約1968年製"

For another reference 6240 Daytona, please see Ultimate Daytona by Pucci Papaleo, pages 324 and 325. 

The manual Daytona reference 6240 was first introduced in the mid 1960s. The reference was upgraded with screw down pushers, making the watch totally waterproof. In addition, the watch was further highlighted with a black acrylic bezel. The present example feature a 1.6 million serial number with original pushers and insert. The dial itself is known as a sigma dial and was fitted with white gold numerals. The sigma dial was not introduced by Rolex until the early 1970s therefore the present dial is from a later period than the watch itself.