Short answer
The Portofino Automatic 37 is a 37 mm steel watch from IWC. References start with IW4581. Judge a copy against four numbers: case diameter 37.0 mm, case thickness near 9 mm, beat rate 28,800 vph, power reserve about 42 hours. A copy that misses those numbers fails on a bench in under a minute.
Specs to measure
- Case: 37.0 mm across, crown excluded. Measure at four points and average.
- Thickness: 9.0 to 9.3 mm. Thin copies use a caseback about 0.5 mm shallower.
- Lug width: 18 mm. Strap tapers to 16 mm at the buckle.
- Weight: about 50 g for the bare head in steel.
- Water resistance: IWC rates the Portofino 37 at 3 bar. A copy stamped 5 bar or 10 bar uses a different case.
Movement check
IWC caliber 35111 sits on a Sellita SW300-1 base. The numbers: 25 jewels, 28,800 vph, 42 hour reserve, 25.6 mm diameter, 3.6 mm height. Read the rotor or open the back. A 21,600 vph movement is wrong. So is a 38 hour reserve. The date should snap near midnight, not drift across 40 minutes.
Dial and hands
Check three areas. First, the printed line at 12. Letter height is about 0.8 mm and the strokes stay sharp at 10x. Second, the date window at 3. The cut edge is square, the aperture sits on the crown axis, and the disc font matches the dial print. Third, the leaf hands. Edges are polished, the center crease is straight, and the seconds hand stops at the minute track without touching it.
Case and finish
The bezel is thin. Brushed surfaces run horizontal on the flanks. Polished surfaces sit on the bezel and the lug tops. The two finishes should meet on a hard line, not a soft blend. The crown is signed and fluted, about 5 mm across. The caseback is closed steel with an engraved reference. Engraving depth varies by factory, so compare the font shape, not the depth alone.
What copies rarely match
Case finishing under a loupe, gasket quality, and rate stability over weeks. Ask for a timegrapher print in six positions and a movement photo in the same frame as the caseback serial. A healthy SW300-1 holds within +/- 10 s/day. Anything wider points to a worn or mismatched movement.