The short answer
3588420 is not a Jaeger-LeCoultre model reference. JLC model numbers carry a Q prefix and run seven digits, which is why you see Q1368420 or Q1358420 on catalogue pages and boutique tags. A bare seven digit string like 3588420 is nearly always a case serial, the stamp that identifies one individual watch. When a listing leans on that number and never names a model, treat it as a flag. There is no spec sheet to check it against.
Reference versus serial, in plain terms
Two numbers live on a JLC caseback and they do different jobs.
- The reference names the model. Master Ultra Thin Moon 39 is Q1368420, and every watch of that model shares it.
- The serial names the individual watch. It is unique, and it is the number the factory logs for service.
Fake listings blur the two on purpose. A number like 3588420 gets pasted into a title because it looks official and because it was copied off a photo of a genuine caseback. Search it and you land on forum threads, not a product page. That is the tell.
What gives a fake away
The movement
Jaeger-LeCoultre builds its own calibers in Le Sentier. On a Master Ultra Thin Moon you get an in-house automatic with a gold rotor, perlage and Geneva stripes on the bridges, and the maker's name engraved rather than printed. Fakes wrap a stock movement in a decorated plate and hope nobody looks. Check the rotor weight, the crispness of the engraving, and how the crown winds. A gritty feel and a rotor that spins like a loose coin is not JLC work.
Dial and moonphase
Look at the printing first. Genuine indices are applied and sit proud of the dial. The date track on the moonphase models is a curved arc, and the spacing between numerals stays even all the way around. On fakes the print is heavier, the moon disc sits flat instead of showing depth, and the arc runs wide at one end. The subdial hands should be thin and sharply cut, not blunt.
Case, lugs, and finishing
The steel Master Ultra Thin Moon is 39 mm across and just under 10 mm thick, so it slips under a cuff. Replica cases often run a touch thicker and the lugs feel slabby. Run a finger along the lug edges. Genuine JLC breaks the edge with a light chamfer, and the brushed and polished surfaces meet on a clean line. Fakes leave a burr or a rounded, soft transition.
Caseback engraving
Depth matters more than wording here. Factory engraving cuts into the metal with sharp shoulders. Replica engraving looks shallow and slightly stacked, and the lettering drifts. Read the model name slowly. Misspelled or squeezed type is common, and so is a reference number placed where the serial belongs.
Papers, box, and the price test
A genuine steel Master Ultra Thin Moon sells in the five figures at retail, and it holds a good chunk of that used. Anyone offering a "new" one for four figures is not doing you a favour. Ask for the Q reference, the serial, the original invoice, and the service record. A watchmaker can open the back in ten minutes and confirm the caliber number, which ends the argument faster than any photo.
Questions to ask before money moves
- What is the full Q reference number?
- What is the serial, and does it match the paperwork?
- Can I see the movement, or a photo of the movement, before I pay?
- Is there a service history from an authorized workshop?
- Will you hold the watch while my watchmaker inspects it?
If you collect replicas on purpose
Do the same homework. Know the reference you want, know the diameter and thickness, and know which caliber sits inside the real thing. A seller who cannot give you a Q number is selling you a guess, and you will feel it on the wrist long before anyone else notices.