Short answer

A fake Master Ultra Thin is a counterfeit of the Jaeger-LeCoultre Master Ultra Thin line. The line sells on one number: case thickness. Genuine references measure 4.05 mm to 9.9 mm, depending on the model. Most fakes run 1 mm to 3 mm thicker. A digital caliper on the caseback settles the question in under a minute.

Genuine dimensions

  • Master Ultra Thin 1907: 38.5 mm wide, 4.05 mm thick, hand-wound caliber 849.
  • Master Ultra Thin Date: 40 mm wide, 7.85 mm thick, automatic caliber 899.
  • Master Ultra Thin Moon: 39 mm wide, 9.9 mm thick, automatic caliber 925 with a 70-hour power reserve.

Jaeger-LeCoultre publishes these figures. Measure the case with the crystal included, because that is how the brand measures.

Tells on a replica

  1. Movement height. A Miyota 9015 is 3.9 mm thick. A Seagull ST18 is 4.0 mm. Add a rotor and a caseback and the stack pushes past 9 mm. The 1907 has no room for that. Replica 1907 watches run 3 mm thicker than the real watch.
  2. Caseback. Genuine Master Ultra Thin watches carry a sapphire back on most references. The movement shows cotes de Geneve stripes, blued screws, and an engraved balance cock. Replica plates are plain or stamped.
  3. Engraving. The caseback holds a reference number, a serial number, and the 1000 Hours Control stamp. JLC tests each watch for 1000 hours. Counterfeit engraving is laser-etched and sits off the caseback curve.
  4. Dial printing. The Jaeger-LeCoultre wordmark sits under the 12. Replica printing shows ragged edges at 10x magnification.
  5. Moon phase. The genuine disc carries a printed moon and stars. Replica discs show a blurred moon and an aperture cut off center.

Price bands for fakes

Three tiers: $80 to $150 buys quartz or a low-grade automatic with mineral glass. $250 to $450 buys a Miyota 9015 or a clone with sapphire. $600 and up buys a Swiss movement claim with no documentation. No tier matches the genuine movement architecture.

Legal status in the US

18 U.S.C. 2320 covers trafficking in counterfeit goods. U.S. Customs and Border Protection seizes counterfeit watches at import. The mark owner can sue for damages. Reselling a watch with a counterfeit mark is a federal crime, not a civil matter.