Short answer
On nearly every listing I have looked at, "31" is not an IWC reference number. It is a seller or factory batch code for one run of replica Portugieser Automatic watches, and it usually points at the movement version inside that run. Ignore the number. Read the spec sheet instead: case diameter, movement name and jewel count, crystal material, dial layout, and strap. Those five lines tell you far more about what will show up in the mail than any batch tag.
What the genuine watch actually is
IWC's Portugieser Automatic is a big, clean dress watch. The classic ref. 5007 runs 42.3 mm across and about 14.5 mm thick, with small seconds at 9 o'clock and a power reserve indicator at 3. The newer Portugieser Automatic 40 is a smaller 40.4 mm case with a simpler dial and no reserve track. Both use an in-house calibre from the 52000 family with a seven day power reserve and Pellaton winding. That matters for buyers because the dial layout is the hardest thing to fake well, and the case size tells you immediately whether a listing is describing the 42 mm or the 40 mm watch.
Why listings use a number like 31
Sellers reuse codes to track which factory built a batch and which movement went inside. Sometimes the number tracks a jewel count on the movement spec sheet. Sometimes it is just an internal SKU. Two listings with the same "31" tag can ship watches from different workshops, with different dials, months apart.
So the number is a starting point, not a promise. If a seller cannot tell you the case diameter and the movement name behind that code, that is your answer right there.
Movement: where the money goes
The movement decides how the watch feels on the wrist and how long it lasts.
- Entry builds use a basic Asian automatic, often noisy rotor, weak power reserve, and a caseback that hides the mechanism behind a solid or poorly printed plate.
- Mid-tier builds copy the rotor engraving and use a decorated automatic with a smoother sweep. Reserves land closer to 40 hours than seven days.
- Top builds replicate the twin-barrel look and the winding feel, but they are still not a seven day movement. Genuine reserve specs are a genuine engineering cost, and no replica at typical prices matches them.
Ask what the reserve is in hours. A seller who says "seven days" is guessing. A seller who says "about 38 to 42 hours" is being straight with you.
Dial and case checks
This is where a Portugieser replica either convinces or falls apart. Look for applied numerals with crisp edges and no glue haze under them. The printed minute track should be thin and even, not thick and soft. On a small seconds model, the subdial should sit at 9 with the hand centered, not drifting toward 8 or 10. The power reserve track, if present, should be printed, not a sticker, and the hand should travel smoothly when you wind it.
Case finishing matters too. The original has a broad polished bezel, slim lugs, and a mix of brushed and polished surfaces. Look for sharp lug edges and a crown that sits flush, not one that wobbles or grinds.
Sizing and wear
A 42 mm Portugieser wears large. It is thin for its size, so it slides under a cuff, but the dial is wide and mostly empty by design. If your wrist is under about 6.75 inches, the 40 mm version is the better shape. Buyers who order the 42 mm expecting a normal dress watch often find it overwhelming. That is not a flaw in the replica. It is the original doing what it does.
Straps, clasps, and hardware
Most replicas ship with a decent alligator-pattern leather strap and a folding clasp. Expect the strap to be the weakest part. Swap it for a real leather strap in 20 or 21 mm and the whole watch looks better. Check that the spring bars are solid and that the clasp locks without play.
Questions worth asking before you buy
- What is the exact case diameter and thickness?
- Which movement, how many jewels, and what is the measured power reserve?
- Is the crystal sapphire with a proper anti-reflective coating on both sides?
- Are the numerals applied or printed?
- Can they send photos of the actual unit, not catalog shots?
One more thing. Counterfeit watches are illegal to sell in the US and most of Europe, and customs seizures happen. Know the rules where you live before you spend. A guide can tell you what to inspect. It cannot make the paperwork problem go away.