ok, hier mal die Beschreibung (Quelle: The Rolex Report).

5 digit Case Ref. nbr. The first 3 digits describe the Watch Type. The second from the last describes the Bezel Type (with some exceptions, especially on more recenst models), and the last number describes the Case Material Configuration ot the watch.
In 2000 changed to 6 digits by adding an extra '1' before the previous number e.g. 16523 was renumbered as 116523.

Frank