Short answer
Patek Philippe made the reference 1518 from 1941 to 1954. Reports put total production at about 281 watches. Four are known in stainless steel. The rest are yellow gold and pink gold. A fake 1518 is one of three things: a modern watch with a 1518-style dial, a genuine movement dropped into a new case, or a real 1518 with a swapped dial.
What the genuine 1518 carries
Movement
Caliber 13''' 130 Q. The base is a Valjoux 23 with a perpetual calendar module on top. A column wheel runs the chronograph. The calendar tracks leap years and needs no date correction until 2100.
Replica 1518 watches use a Valjoux 7750 or a Chinese copy of it. The 7750 runs a cam and a lever, not a column wheel. Its calendar is a simple module. Watch one through February and the date lands on 29 in a non-leap year or stops at 28.
Case
Diameter is 35mm. Patek bought cases from outside makers. The inside caseback carries the maker mark plus Swiss precious metal hallmarks. A fake case has no hallmark, a hallmark struck with the wrong punch shape, or laser engraving where a stamp belongs. A gold 1518 case weighs more than gold plate over brass.
Dial
Subdials sit at 3 and 9. Day and month windows sit at 12. A moon phase and a date hand sit at 6. Indexes are applied gold. Printing is thin and sharp under a loupe. Replicas show four faults: subdial spacing that crowds the center, a moon disc printed in flat gray, a date hand that jumps at the wrong hour, and dial text with soft edges.
Price
A genuine 1518 sells between about $300,000 and $1,500,000 at auction. One stainless steel example sold in Geneva in November 2016 for CHF 11,002,000. A replica costs $300 to $900. No seller offers a working perpetual calendar chronograph from 1948 for $600.
Checks that settle the question
- Request an Extract from the Archives from Patek Philippe. It lists the movement number, the case number, and the case metal.
- Match the movement number on the bridge to the case number on the caseback.
- Ask for a service record. Patek Philippe in Geneva keeps history for watches it has handled.
- Press the chronograph. A column wheel starts the seconds hand with no jump. A 7750 kicks.
- Compare dial print to a published photo of a documented 1518.
Legal position in the US
Counterfeit watches fall under 18 U.S.C. 2320. Import, sale, or transport for sale is a federal offense. A first offense by an individual carries up to 10 years in prison and a fine. Customs and Border Protection can seize shipments under 19 U.S.C. 1526. Owning one is not the crime. Import and sale are.