Short answer: the replica Portofino moon phase 35 stands or falls on two things, the moon disc gearing and the case proportions. Nail those and the rest is finishing detail. Miss them and you are wearing a watch whose moon completes a full lap every 24 hours, which is the most common flaw I see on these listings.
What the 35 in the listing actually means
IWC catalogues its Portofino moon phase in this size class at 37 mm, not 35. When a seller writes 35, they usually mean the case measured across the back without the crown, or a rep-specific case that has been trimmed slightly. Ask for a photo with calipers on the case flank. The number matters because the dial layout is drawn for a wider aperture. Shrink the case and the moon window, the date cutout and the applied indices start crowding each other, and that crowding is visible on the wrist even to someone who has never handled the real thing.
The moon phase is the whole point
A true moon phase runs on a 29.53 day cycle, which watchmakers implement with a 59 tooth disc that clicks forward once a day. Cheap reps cheat. They fit a 24 hour disc painted with two moons, so the display spins all the way around in a day. Test it before you pay. Set the moon to full, photograph the aperture, then look again 24 hours later. On a correct movement the disc has crept forward by roughly one tooth and the change is barely visible. On a fake moon phase the disc has jumped a twelfth of the way round.
While you are in there, check the printing. Good discs show crisp white moons on a deep blue field with stars that do not bleed. Bad ones look like a printed sticker under a loupe, and the aperture edges are rough where the plate was stamped.
Movement and caseback
The genuine Portofino moon phase 37 uses calibre 35800, which is a Sellita SW300 base with a moon phase module on top. Replicas in this size class tend to run one of three things: a decorated Chinese automatic with a module, a Seagull ST series, or a Miyota 9015 with an add-on plate. None of those are bad if they are honest about it. Ask for a movement photo and a timegrapher readout. You want to see amplitude above 250 degrees and a rate within plus or minus 15 seconds a day. Also check the engraving on the rotor and the perlage on the bridges. Laser etching looks flat and grey next to proper finishing.
Case, dial and the tells
- Case thickness. A moon phase module adds height, so a rep that comes in under about 10.5 mm is probably running a fake moon disc.
- Lugs. The Portofino curves down sharply. Straight or slab-sided lugs are the giveaway at arm's length.
- Hands. Slim dauphine shapes, polished bevels, no rounded tips.
- Date font. Compare the numeral shape against a reference photo. Reps often use a generic font that is a touch too bold.
- Crystal. Sapphire with double sided anti-reflective coating. Tilt it under a lamp and the dial should almost vanish.
- Strap. Alligator grain embossed on calf is normal at this price, but the stitching should be even and the buckle engraving clean.
Questions to put to the seller
- What is the exact case diameter with the crown, and can you send caliper photos?
- Does the moon disc advance once per day or once per 24 hour revolution?
- Which movement, and can you send a timegrapher screenshot?
- Are the photos of the actual watch or a stock image?
- What is the water resistance rating, and was it actually pressure tested?
Price and expectations
Under about 150 dollars you are buying a printed moon and a stamped dial. The useful band sits roughly between 250 and 450 dollars, where you get a real moon disc, sapphire and decent hand finishing. Above that, you are paying for a specific factory name rather than a better watch. Do not expect service support. Treat it as a piece you enjoy and replace, not an heirloom, and keep it away from swimming pools whatever the caseback says.
Bottom line
Buy the moon phase first, the case second, the finishing third. If the disc behaves like a real one and the case measures close to 37 mm on the flank, you have found a good example. Everything else is negotiable.