Short answer
There is no 35 mm Portofino moon phase in the IWC catalog. IWC lists the Portofino Automatic Moon Phase 37 at 37 mm and the Portofino Automatic 34 at 34 mm. A watch sold as a "Portofino moon phase 35" is a replica with a case that measures about 35 mm across, taken at the bezel and not counting the crown. Treat 35 as a measurement, not a reference number.
What the genuine watch does
The Portofino line dates to 1984. The Automatic Moon Phase 37 uses a stainless steel case, a sapphire crystal, and an automatic movement with a moon phase disc. The disc steps one tooth each day on a gear train built around the synodic month, which averages 29.53059 days. A simplified 29.5 day ratio drifts by one day about every 33 months and needs a reset. IWC lists caliber 35800 for the Automatic Moon Phase 37.
How replica moon phases work
Three builds appear in replica Portofino moon phase watches.
- Printed disc. The moon is a decal. It does not move. This is the cheapest build.
- Day and night disc. A 24 hour wheel turns a two tone disc. The cycle is 24 hours, not 29.5 days.
- 29.5 day disc. The moon phase runs off the hour wheel through a reduction gear. Cycle length lands within a day or two per year of the genuine watch.
Ask the seller for two photos of the disc taken 24 hours apart. A real moon phase moves about 1/29.5 of a full turn. A day and night disc moves half a turn. That single test settles the build type.
Case, movement, and fit
A 35 mm case sits between the two genuine sizes. Case width alone does not predict fit. Lug to lug distance and case thickness matter more, so ask for both numbers in millimeters. Strap width on replicas of this family is 18 mm in most samples. Better replica movements run at 28,800 vibrations per hour, which is 8 beats per second, and use a 2824 style automatic. Cheaper builds run at 21,600 vibrations per hour, which is 6 beats per second, and show a slower sweep.
Legal status in the United States
Counterfeit watches are illegal to import and sell in the US. 18 U.S.C. 2320 covers trafficking in counterfeit goods and carries up to 10 years for a first offense. The Lanham Act gives trademark owners civil claims. Customs and Border Protection seizes counterfeit watches at ports of entry. A replica is not a genuine IWC product. It carries no IWC warranty and no IWC service.