Short answer

Rolex has no reference 1666. The catalog uses four, five, and six digit numbers, and 1666 is not one of them. A watch listed as a "Rolex 1666" is mislabeled or a replica. The closest real number is 16660, the Sea-Dweller called the Triple Six. The trailing 6 in the search may also point to the 6 o'clock side of the case, where Rolex stamps the serial number and places the etched crown on the crystal.

What the digits on the case mean

Two sets of digits sit on the case between the lugs, hidden by the bracelet. The reference number goes at 12 o'clock. The serial number goes at 6 o'clock. From the mid-2000s, Rolex repeated the serial on the rehaut, the inner ring around the dial. A watch with no readable reference at 12 o'clock and no readable serial at 6 o'clock cannot be verified.

Three marks at 6 o'clock

  • Laser etched coronet on the crystal. Rolex added this mark around 2001 on sapphire crystals. It sits at 6 o'clock, small and faint, and shows at an angle in light. Replicas cut it too deep or too wide.
  • Serial between the lugs. The engraving is shallow, even, and spaced the same from letter to letter. Fake cases show deep stamps, uneven spacing, or a raised burr around the digits.
  • Rehaut ring. On later cases the rehaut carries the word ROLEX plus the serial. Letters line up with the minute track. Misalignment is common on replicas.

16610, 16660, and the 1666 problem

The Submariner Date 16610 ran from 1989 to 2010. It has a 40 mm case, an aluminum bezel insert, caliber 3135, and 300 m water resistance. The Sea-Dweller 16660 ran from 1980 to 1988, uses caliber 3035, and is rated to 1,220 m with a helium escape valve at 9 o'clock. If a seller names 1666, ask for a photo of the case between the lugs. No photo means no sale.

Red flags

  • Reference number absent from Rolex literature.
  • Display caseback on a Submariner or Sea-Dweller. Rolex uses solid steel backs on both lines.
  • Cyclops magnification under 2.5x. Rolex uses 2.5x on date models.
  • Bracelet end links that gap at the case. Original 93250 links fit flush.
  • Weight below the normal range. A steel Submariner with bracelet lands near 130 g to 140 g on a kitchen scale.
  • Movement marked ETA, Seagull, or a clone stamp. Period correct Rolex calibers are 3035, 3130, and 3135.

How to confirm one

Ask a watchmaker to open the caseback. Caliber 3135 has 31 jewels, runs at 28,800 vibrations per hour, and offers a quickset date. Rolex does not publish a public serial database. Printed cards and third party tables have gaps. Service records from a Rolex service center carry more weight than any card. If the seller cannot show the case engravings or the movement, treat the watch as a replica.