Short answer

306.104 does not appear in the reference numbers A. Lange & Söhne publishes for the Zeitwerk. That line lives in the 140.xxx block, and the Zeitwerk Date sits in the 148.xxx range. So a watch sold as a "Zeitwerk 306.104" is already telling you something: the number is decoration, not documentation. People searching that exact string are almost always looking at a counterfeit listing. The good news is that the Zeitwerk is close to impossible to fake well. Ten seconds with a loupe usually ends the conversation.

What the real Zeitwerk does

Lange introduced it in 2009 as a mechanical answer to a digital clock. Two large apertures carry the time: hours on the left, minutes on the right. Both numerals jump. There are no hour or minute hands anywhere on the dial. A small seconds subdial sits at the bottom.

The trick is not the display. It is the energy. A hand-wound movement from the L043 family uses a constant-force escapement, a remontoir that meters out power in steady pulses and releases one pulse each minute to snap the discs forward. Without that stored release, the jump would crawl, or it would stall. That is the whole watch, and it is why the mechanism resists cloning.

Why copies fall apart

A counterfeit at a few hundred dollars has no remontoir. It has a printed disc, a quartz module, or a tiny digital screen behind a smoked window. You can catch the difference without opening the case:

• The jump. Real ones snap at the minute change. Copies drift, creep, or lag a few seconds behind.
• Alignment. On a genuine Zeitwerk each numeral sits square inside its aperture. Fakes are off by a hair, and it shows at the edges.
• The dial. Lange numerals have real depth and finishing. Copies look flat and slightly waxy under light.
• The case. Check the lugs and the crown. Genuine Lange lugs hold a crisp bevel. Replicas round them off.
• Engraving. The balance cock engraving on a real Lange is freehand work. Fake engraving looks stamped and repeats itself.
• Weight and sound. A hand-wound Lange feels dense in the hand. Cheap copies feel hollow, and winding is gritty.

Listing patterns that give it away

I look at how the seller writes the reference. Real Lange listings name the reference and the metal, like a white gold or pink gold Zeitwerk, and they show the movement. Fake listings lean on the model name plus a number that sounds official, pair it with stock photos, and skip the caseback. Other tells: a price that undercuts steel sports watches, no serial photo, a seller who answers questions about the remontoir with a paragraph about "Swiss quality," and a return policy that vanishes once money moves.

One more thing worth knowing. Counterfeit watches are illegal to import into the United States, and customs can seize them. That risk sits with the buyer, not the seller, and no listing photo changes it.

If you are actually shopping

Cross-check any reference against the published Lange catalog before you send money. If the number does not exist there, stop. If it does exist, ask for a photo of the movement, a photo of the caseback engraving, and a photo of the papers with matching numbers. Buy from a dealer who has handled Lange before, and expect to pay real money, because the constant-force mechanism is the reason the watch costs what it costs. There is no shortcut version. Anyone promising one is selling you a display case with a story attached.