First, the answer: "3399" is not an Audemars Piguet reference number. It is a trade code that some sellers and factories in the Chinese replica market attach to a specific build of the Royal Oak Perpetual Calendar. AP's own perpetual calendar references run in the 26574, 26579 and 26674 families. So if a listing leads with "3399" and never names the watch it copies, treat that as a sign the seller is talking to people who already know the shorthand, not to someone who needs the basics.

What the replica is actually copying

The target is the Royal Oak Perpetual Calendar, usually the 41mm stainless or rose gold version with the blue or black grande tapisserie dial. The real thing carries AP calibre 5134, an ultra-thin automatic with a perpetual calendar module: day, date, month, leap year, plus moon phase and a 24-hour subdial. The layout is the hard part to fake. Four subdials, correct spacing, a date that sits at 3 o'clock, and a moon phase at 9. Get the subdial positions wrong and the watch reads as wrong from across a table.

How the movement usually works

No replica at this level ships with a true perpetual calendar. Most run a modified automatic base, often a 7750-derived or thin Asian micro-rotor clone, with a calendar module bolted on top. That means the calendar advances but does not truly account for leap years the way the genuine 5134 does. Some builds let you set month and leap year by pusher, some do not. Ask directly. If a seller claims "full perpetual function" with no explanation of how February is handled, that claim deserves skepticism.

What I would check before paying

• Case thickness. Genuine is about 9.5mm. Anything over 11mm is a different animal on the wrist.
• Subdial hand length and finish. Look for hands that reach the markers, not stubby ones.
• Dial texture. Grande tapisserie should have crisp raised squares, not soft bumps.
• Bracelet taper and end-link fit. This is where cheap builds fall apart.
• Whether the pushers actually do anything, or are decorative.

The legal reality in the US

Counterfeit watches are illegal to import and to traffic in the United States. Customs can seize a shipment, and selling replicas as genuine carries real exposure under trademark law. That is not a scare tactic, it is the baseline. Buying one for personal use sits in a gray zone that varies by jurisdiction, and the practical downside is that you have no recourse if the watch arrives broken, wrong, or never arrives at all.

Questions worth asking

Ask for a video with the time set to a known date so you can watch the calendar change. Ask which factory built it and which movement is inside. Ask about water resistance and whether the caseback gasket is real. A seller who answers all three in specifics is a different risk than one who replies with a photo and a price. The code 3399 tells you almost nothing on its own. The answers do.