The replica IW371748 lives or dies on three things: case thickness, subdial spacing, and the shape of the leaf hands. Get those right and you have a convincing copy of the IWC Portugieser Chronograph. Get them wrong and no amount of polished finishing will hide it. I check thickness with calipers before anything else, because it is the one flaw you can spot from across a table.
What the IW371748 actually is
It belongs to the 3717 generation of the Portugieser Chronograph: a 40.9 mm steel case, a silver-plated dial with the chronograph counters stacked at 12 and 6, running seconds at 6, a date window at 3, and IWC's calibre 79350 inside. That movement is a Valjoux 7750 that IWC reworked, which matters for replicas. The genuine article is thin for a 7750 watch, roughly 12.3 mm, and that thinness is the hard part to copy.
The three make-or-break details
Case thickness
Most budget copies sit between 13.5 and 14.5 mm. On the wrist that reads as a puck, and the case flanks look slab-sided. A good replica lands at 12.5 to 13 mm. Ask for a caliper shot with the caseback on. If a seller will not send one, move on.
Subdial spacing
The two counters should sit close to the center, leaving a wide, clean outer track. Cheap dials push them outward, which crowds the numerals and kills the whole look. Compare the gap between the 12 subdial and the printed minute track against a reference photo. This is the detail that separates a $250 copy from a $500 one.
Hands
Feuille hands, leaf-shaped and tapered to a point. Replicas often ship with dauphine hands or leaves that are too wide at the base and blunt at the tip. The seconds hand on the 6 counter should be a thin needle, not a paddle.
Movements you will find
Three options show up in this reference. The Asian 7750 is the workhorse: correct 12-6-9 subdial layout, running seconds at 9 built in, and it accepts the date at 3 without a mod plate. It is also the thickest. Clone 79350 movements exist but are rarer and pricier. Quartz versions are out there and they are usually obvious because the chrono seconds ticks instead of sweeping.
The 7750 has a rotor wobble and a slightly noisy winding feel. That is normal, not a defect. What is a defect is a chrono hand that snaps back to zero off-center or a reset that sticks.
Dial, crystal, and strap
Look for applied numerals with crisp edges and no visible glue. Printing should be sharp under a loupe, especially the small "IWC" and "Schaffhausen" block. The crystal should have double AR coating; a bluish smear across the dial under a lamp means the coating is single-sided or absent. Straps on decent copies are alligator-grain calf with a steel pin buckle stamped cleanly. The buckle is a small tell, and it is cheap to replace.
What to pay and what to skip
Expect to pay more for correct thickness and a serviced 7750. Skip anything with a 14 mm case, blurry printing, or a date font that looks like a different typeface from the rest of the dial. Skip sellers who send only stock photos. And skip watches advertised with 100 m water resistance. The genuine watch is rated to 30 m, so a higher claim tells you the seller is guessing.
Treat any replica as costume jewelry with a movement. Have the gaskets checked before you wear it near water, and budget for a service if the chrono feels gritty.