Short Answer
A fake Master Ultra Thin is a replica of the Jaeger-LeCoultre Master Ultra Thin, a dress watch line with case sizes from 34mm to 40mm. Replicas copy the case diameter, the dial layout, the dauphine hands, and the applied indices. They do not copy the JLC movement. The gap shows in case thickness, in caseback engraving, and in the finishing seen through the sapphire back.
What the Genuine Watch Is
Jaeger-LeCoultre used the Ultra Thin name in 1907 for a pocket watch. The modern Master Ultra Thin collection dates from 2011. The line runs on thin manual and automatic calibres, with sapphire crystal on the front and the back.
- Case sizes: 34mm, 38mm, 38.5mm (Master Ultra Thin 1907), 39mm (Moon), 40mm (Date).
- Thickness: 4.05mm on the Master Ultra Thin 1907, about 7.5mm on the 38mm automatic, about 9.9mm on the Moon 39. Numbers move by reference and by year, so confirm the exact reference.
- Movements: calibre 849 (manual), 896 and 899 (automatic), 925 (automatic with moon phase).
- Water resistance: 50m on most references. Strap: alligator leather.
Where Replicas Match
Most replicas get the case diameter right and the dial layout close. Date window placement, moon phase position at 6 o'clock, and the dauphine hand shape are copied from photos. Strap color and buckle style follow the original.
Where Replicas Differ
- Thickness. Replica cases often run 1mm to 2mm thicker, because the movement inside is thicker than calibre 896.
- Movement. Common fits are a Miyota 9015, a Seagull ST series, or a Chinese automatic with a decorated rotor. Beat rates land at 21,600 or 28,800 vph.
- Caseback. Engraving is shallow. Font weight, spacing, and the JLC script differ from the original.
- Dial. Text print, index polish, and moon phase disc artwork are the weak points.
- Glass. Some units use mineral glass or sapphire with weak anti-reflective coating.
Checks Before You Buy
- Ask for a photo of the case on calipers, measured at the caseback edge.
- Ask for a video of the seconds hand and a clear photo of the movement.
- Compare the reference number on the caseback against the JLC catalog entry.
- Check the crown logo relief and the strap hardware.
- Weigh the watch. A gold model weighs more than a steel model of the same size.
Known Limits
No replica carries a JLC calibre. Service and parts come from the replica seller, not from Jaeger-LeCoultre, and the brand will not service a replica. Resale value sits far below the price of a genuine watch. Thickness figures change across references and production years, so measure the exact watch in front of you instead of trusting a listing.