Short answer
Cartier does not list a 21mm Clé de Cartier. Public catalog data shows the collection in 31mm, 35mm and 40mm case diameters. A listing marked "21mm" measures something other than the case, or names a size the brand does not publish. Ask the seller which part of the watch the number describes before you use it to judge a replica.
What the genuine Clé de Cartier is
Cartier released the Clé de Cartier in 2015. The name refers to the crown, which sits in a slot and turns on a key-like axis. Core specs from brand and press material:
- Case diameters: 31mm, 35mm, 40mm.
- Metals: steel, rose gold, yellow gold, white gold, some with diamond-set bezels.
- Movement: caliber 1847 MC automatic on many references, about 42 hours of power reserve.
- Crystal: sapphire. Case back: sapphire on models with the 1847 MC.
- Dial: Roman numerals, blued steel sword hands, small hidden Cartier signature on some references.
Where a 21mm figure comes from
Numbers in replica listings mix three measurements. Check each one:
- Case diameter, read across the case at 9 and 3 o'clock with the crown excluded.
- Lug width, the gap between the lugs where the strap meets the case.
- Strap width at the buckle end.
A 21mm figure sits near the strap and lug widths of several mid-size watches. It may describe the band, not the case. Without calipers on the actual watch, the number stays unverified. Treat any listing that quotes one figure for both case and strap with caution.
Checks to run on any replica
- Measure the case with calipers. A 31mm Clé should read 31mm at the widest point, crown aside.
- Confirm the movement type. Quartz and automatic versions of the same listing sell at different prices.
- Turn the crown. On a Clé the crown sits in a slot and should rotate on that axis without wobble.
- Weigh the watch. Steel and gold replicas of one size differ in mass.
- Read the engraving on the case back under magnification. Stamped text is common on replicas. Laser engraving is deeper and cleaner.
- Question water resistance claims. Most replica cases are not pressure tested, and a rating printed on a case back proves nothing.
Legal status in the US
19 U.S.C. § 1526 bars import of goods that copy a registered trademark. U.S. Customs and Border Protection can seize shipments of counterfeit watches. Sellers face civil liability. Buying one watch for personal use is a different matter from resale, but seizure risk at the border stays.
What photos cannot show
A photo cannot confirm case diameter, movement type, water resistance, or metal content. Ask for caliper readings, a movement shot, and a weight in grams. If a seller cannot supply those three items, the "21mm" label and the rest of the listing remain unconfirmed.