Short answer: 3588420 is not a Jaeger-LeCoultre reference number. Jaeger-LeCoultre publishes references in two shapes. Modern pieces use a Q plus six digits, like Q1368420 or Q1368430. Older pieces use dot-separated case numbers, like 136.8.42 or 140.8.89. Seven bare digits with no Q and no dots fit neither pattern. The number you are chasing looks like a stripped or mistyped version of Q1368420, the 39mm Master Ultra Thin Moon in stainless steel.

That matters because people search it, land on a listing page, and assume the number came from somewhere official. It did not. There is no known counterfeit batch stamped "3588420." There are counterfeit Master Ultra Thin Moon watches, and there is a messy number in forum threads and marketplace titles. Search engines glued the two together.

Why a near-miss reference is the red flag, not the number itself

When a seller writes a reference that is almost right, one of three things is happening. They typed it from memory. They copied it from a bad listing. Or they built the listing around a replica and padded it with plausible digits. The first two are sloppy. The third is the one that costs you money.

I have watched this pattern for years on grey-market pages. The listing photo is sharp, the price is 30 to 40 percent under the going rate for a steel Moon, and the reference has a digit out of place. That gap between price and reference is the tell. Nobody sells a genuine Q1368420 in good shape for a third off and then fumbles the model number.

How a Master Ultra Thin Moon fake usually fails

Caseback and movement

Ask for a straight-on caseback photo before anything else. Genuine JLC casebacks are engraved, not laser-etched, and the engraving sits deep in the metal with crisp edges. Fakes tend to look shallow and slightly fuzzy at the corners. The caliber 925 that drives the Moon is finished to a standard replicas do not reach: Geneva stripes with clean borders, blued screws that are actually blued steel instead of painted heads, and a rotor that spins with weight. A hollow-sounding rotor is a bad sign.

Dial, moonphase, and hands

Look at the moon disc. On the genuine watch the stars are sharp points and the moon has a soft, even finish. On most fakes the stars smear and the moon looks printed rather than applied. Check the applied indices too. They should sit flush with a mirror polish on the sides and a brushed top. Then look at the date window, which on the Moon sits at 6 with the moonphase. Fake versions often let the date disc sit a hair off center.

What to ask before money moves

• The full reference with the Q prefix, plus a caseback photo that shows the same digits.
• A movement photo or a short video with the rotor turning.
• Original papers with a serial that matches the case.
• A return window in writing, long enough for an independent watchmaker to open it.

One more thing worth saying out loud. If you are shopping replicas on purpose, fine, that is your call and the price should reflect it. What you cannot do is pay replica money for a watch sold as genuine, or sell one on as real. That is fraud in every US state, and the watch world is small enough that it follows people around.

Verify the reference first. Everything else follows from that.