Quick answer

A genuine Rolex 16660 Sea-Dweller uses a 1575 movement, a helium escape valve with a specific thread pattern, and caseback engravings that match the reference. Most fakes fail on the caseback depth, the valve operation, and the dial printing. Use the checks below in order.

1. Inspect the caseback engravings

Turn the watch over. The 16660 caseback has a raised center section and engraved text: "ROLEX" and "ORIGINAL GAS ESCAPE VALVE" around the edge. The engraving is crisp and shallow. Fakes often use laser etching that looks too deep, too wide, or uneven. Check the reference number "16660" between the lugs at 12 o'clock. It should be stamped, not engraved, and match the serial at 6 o'clock.

2. Test the helium escape valve

The valve sits on the left side of the case at 9 o'clock. On a genuine 16660, the valve unscrews with about two turns and has a distinct resistance. The valve crown has a flat top with a single dot. Replicas often use a fake valve that does not move or turns without resistance. Do not force it. If the valve feels loose or does not seat flush, treat the watch as suspect.

3. Check the dial and hands

The 16660 dial has a matte finish and tritium lume plots that age to a warm patina. The word "SEA-DWELLER" sits above the depth rating. Fakes often use glossy dials, bright white lume, or oversized fonts. Look at the hands: genuine hands have a slight dome and a polished edge. The sweep second hand should move in small steps, not jump.

4. Verify the movement

Remove the caseback with a proper tool. The 16660 uses a Rolex caliber 1575, a 26-jewel automatic movement. Look for the "1575" stamp and the Rolex crown on the bridge. Fakes use generic movements like an ETA clone or a Chinese copy. These movements lack the correct rotor engraving and jewel count. If you cannot open the case, have a watchmaker do it.

5. Examine the bracelet and clasp

The original 16660 came on a 93150 Oyster bracelet with a 580 end link. Check the clasp code: it should be a two-letter date code that matches the case serial. Fakes often have hollow end links, a flimsy clasp, or a stamped code that does not line up. The bracelet should have 13 links and a diver extension.

6. Look at the rehaut and serial

The rehaut is the inner ring between the dial and crystal. On a genuine 16660, the rehaut is plain, not engraved with "ROLEX." Engraved rehauts came later. The serial number at 6 o'clock is laser-etched on later examples, but early 16660s have it engraved between the lugs. A fake often puts the serial on the rehaut or uses a sticker.