Short answer

The 3940 is Patek Philippe's slim 36 mm perpetual calendar from 1985, and a replica only reads as correct if it copies three things at once: the 36 mm case, the three-subdial layout, and a thin automatic movement under a display back. Get those right and it fools most people at arm's length. Miss one and it looks off immediately, usually because the case is too big or the subdials put the wrong information in the wrong place.

What the real 3940 actually is

Patek launched the reference 3940 in 1985 and built it until roughly 2006, when the 5140 took over. Inside sits calibre 240 Q, an ultra-thin automatic with an off-center micro-rotor in 22k gold. The dial shows the day of the week plus a small 24-hour indicator in the subdial at 9 o'clock, the month and a leap-year aperture in the subdial at 3, and the date with the moonphase in the subdial at 6. The case is about 36 mm wide and under 9 mm thick, which is the whole point. It was made in yellow, rose and white gold and in platinum, with a silvered opaline dial on most examples. Steel versions exist in tiny numbers and auction houses treat them as trophy lots.

Dial details that separate a good replica from a bad one

Subdial layout

This is where most fakes fall apart. On the real watch the 9 o'clock subdial carries the day of the week, with a tiny day and night window cut into it. Plenty of replicas move the 24-hour display somewhere else, or print all four calendar functions on flat discs that never change. Watch the subdial at 3: the month should sit in a ring around a small leap-year aperture showing 1 through 4, not a naked number.

Moonphase and leap year

A working moonphase advances once a month on a 59-tooth disc, and the leap-year aperture on a genuine-spec movement jumps only every four years. On cheaper replicas the moon disc is printed and static, or it drifts. If the seller cannot tell you what the moon disc does, assume it does nothing.

Printing and hands

Look at the signature. Genuine dials use crisp, thin lettering, with even spacing between "Patek" and "Philippe" and a properly formed ampersand. Baton hands should be faceted and dark from the side. Fat, shiny hands and a blurry logo are giveaways that show up in photos long before anyone inspects the caseback.

Case, thickness and metal

The 3940 is a dress watch. It slips under a cuff and sits flat on the wrist, and that flatness comes from a very thin movement. Replicas built on generic automatic bases often come out at 38 to 40 mm and 11 or 12 mm thick, which changes the whole silhouette. If the dimensions in the listing are not stated, ask. Also check the lug shape and how the case flanks are polished, since the original has soft, sloping sides rather than a sharp slab.

Movement: what you are really paying for

Two families show up in replica 3940s. The first is a quartz or decorated mechanical movement with non-functioning calendar discs, and it is cheap for a reason. The second is a clone or modified automatic with a working perpetual calendar module, and the moonphase and leap year actually track. If the price is low and the seller calls it a perpetual calendar, ask for a video of the leap-year aperture changing. The honest answer arrives fast.

Checklist before you pay

• Confirm 36 mm case width, or accept that it will wear larger than the original.
• Ask for the case thickness and compare it to the genuine figure.
• Request photos of the day, month, date and moonphase at different settings.
• Check that the leap-year aperture exists and sits inside the month subdial.
• Look for a display back showing a micro-rotor, not a full circular rotor.
• Read the printing closely for spacing and the shape of the ampersand.
• Ask what happens when you set the calendar, since a fragile module is easy to damage.

What to expect to pay

Genuine 3940s trade in the tens of thousands of dollars, and rare metal or dial variants go far higher. Replicas sit in a wide band, from inexpensive quartz versions with decorative subdials to automatics with a functioning calendar module that cost several hundred dollars. The gap buys you a movement that survives daily wear and a case that matches the original proportions. Decide which of those two matters more before you order, because a cheap replica of a perpetual calendar will not do either one well.