Rolex serial numbers have historically been one of the primary tools used to date and authenticate watches. For much of the brand's history, serial numbers were engraved between the lugs on the case side at the 6 o'clock position, visible only when the bracelet or strap was removed. Starting around 2005 with the M-serial prefix, Rolex began engraving the serial number on the inner rehaut (the ring between the dial and the crystal), making it visible without removing the bracelet. Both locations may feature serial engravings on transitional-era watches.
Rolex serial numbers historically followed a sequential system, with specific number ranges and letter prefixes corresponding to known production years. Collectors and dealers commonly reference serial number charts to approximate the production date based on where the number falls in the sequence. Letter-prefix serials began around 1987 and continued through several letter series. However, around 2010, Rolex transitioned to a randomized serial number system, meaning that serial numbers on newer watches can no longer be used to determine the production year through sequence alone.
The format and quality of the serial number engraving can also serve as an authentication indicator. Genuine Rolex engravings are typically very finely executed, with clean, precise characters. On models produced after approximately 2005, the serial number on the rehaut is often accompanied by the Rolex crown logo repeated around the circumference. Poorly executed or inconsistent engravings are commonly flagged as potential indicators of a counterfeit or aftermarket modification.