This is an extremely rare Rolex Prototype that was once owned by Eric Clapton.
This watch was made in 1965 and measures 39.5mm, so it is the size of a current Daytona. This is unusual because the Daytona models at that time were only 36mm.
It is fascinating that Rolex experimented with creating a second watch with a Nautical theme, the first obviously being the Submariner. It took Rolex another 3 decades before they finally introduced that Yacht Master.
Some people refer to this prototype as the Daytona Yachtmaster because they look so similar. The watch is a Chronograph and has the Rolex proprietary designation for a Chronograph which is "Cosmograph."