(819904) Cartier Santos Galbée Moonphase 18k Yellow Gold

Piece unique.
Approx. 1989 - 1993
Watch only. Will fit up to 6.85” wrist.Recent full quartz service. Running perfect. Unique diamond cabochon/crown.

Typically this moonphase model comes with a blue cabochon and carries the reference 819901. But the caseback on this watch shows 819904. If you search 819904, you’ll only find examples with a diamond-set case and diamond cabochon. This however has a clean gold bezel/case and diamond cabochon combo.

I checked this watch in the Cartier’s system and confirmed that its serial number is tied to the model number WM3002C5, which is the diamond-case variant that matches reference 819904.

Here’s the twist: the model number on this specific watch shows WM30XXXX, and there’s no record of that anywhere. Because 819904 is originally meant for the diamond-case version, the fact that this watch carries WM30XXXX makes it very likely to be a piece unique and or the only one in the world.

1. Normal production pattern
• The “standard” Santos Galbée Moonphase large size was catalogued in the early 90s under ref. 819901, with the usual blue sapphire cabochon.
• This watch has the same architecture but the caseback is stamped 819904. When you search that number you mostly find examples of a factory diamond-bezel version with a diamond cabochon.

2. Cartier’s internal code vs. public reference
• Cartier uses two layers of identifiers. The public reference number (like 819901) is what appears in catalogs.

• Behind the scenes they assign a separate SAP or model code (these WM-series numbers, for example WM3002C5, which corresponds to the diamond-set version of the Moonphase).

3. What I uncovered
• I checked the watch’s serial in Cartier’s own system and it returned WM30XXXX, which does not appear anywhere publicly.

• That means Cartier’s database recognizes this specific watch as a model that doesn’t have a standard catalog listing. It’s essentially an internal “build code” that never got a public SKU.

4. How that points to special-order status
• When Cartier produced a custom configuration—say, building the diamond-spec Moonphase but delivering it without the diamonds or with a different crown—they often created a new internal WM code for that order.

• That’s exactly the kind of situation where a piece ends up being a true special order / pièce unique even though it shares a family reference with regular production.

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