Short answer
The watch sold as a fake Cartier Tank Must 31 mm copies the Cartier Tank Must large. Cartier lists the large model at 33.7 mm by 25.5 mm and 6.6 mm thick. Listings that use 31 mm measure the case at a point that includes part of the crown, or they round the number down. A copy differs from the genuine watch in four places a buyer can check: crystal material, crown stone, dial print, and caseback engraving.
Where the 31 mm Number Comes From
Cartier sells the Tank Must in two main sizes. The small model is 29.5 mm by 22 mm. The large model is 33.7 mm by 25.5 mm. The SolarBeat ships in the large case with a solar movement. No current Cartier catalog entry lists a 31 mm Tank Must. Replica sellers use 31 mm as a label because it sits between the small and large numbers and reads well in a product title. Ask the seller for the case width at 3 and 9 o'clock and the length from lug tip to lug tip. Compare those two numbers to 33.7 mm and 25.5 mm.
Genuine Tank Must Large Specifications
- Case: steel, 33.7 mm x 25.5 mm, 6.6 mm thick
- Crystal: sapphire
- Movement: Cartier quartz, stated battery life 8 years
- Water resistance: 30 m, or 3 bar
- Crown: steel with a blue spinel cabochon
- Dial: silvered opaline, Roman numerals, blued steel sword hands, hidden Cartier signature cut inside the VII numeral
- Strap: calfskin or steel bracelet. The SolarBeat model uses a strap made with plant material
Four Checkpoints
- Crystal. The genuine watch uses sapphire. Many copies use mineral glass, which scratches at lower force. A water drop test does not hold up. A scratch test damages the watch, so skip it.
- Crown stone. The genuine cabochon is a faceted blue spinel set flush in the crown. Copies use glass or plastic, often a flat dome with a visible seam at the base.
- Dial print. The chemin de fer minute track and the Roman numerals on the genuine dial hold a constant line weight. The hidden signature inside the VII is thin and needs magnification to read.
- Caseback. Genuine engraving cuts into the steel with visible depth. Copies use laser etching that reads gray and shallow under light. Reference and serial numbers sit next to the Swiss Made mark.
Quartz Does Not Prove Origin
The genuine Tank Must and the copies run quartz movements that step once per second. A stepping seconds hand tells you nothing. Opening the caseback carries risk: the gasket can be damaged when the back comes off, and that gasket keeps the 30 m water rating.
Price Bands
Cartier listed the steel Tank Must large on a leather strap near $3,300 in the US in 2024. Replica listings run from about $60 to $300. The spread follows the movement, the crystal, and the strap. A $60 copy with mineral glass and a flat printed dial will not match a $250 copy with sapphire and a steel bracelet. Price alone does not confirm quality in either direction.
Questions to Put to a Seller
- Case width at 3 and 9 o'clock, and case length from lug tip to lug tip
- Crystal material, named as sapphire or mineral
- Crown stone material and setting method
- Strap material and the number of stitch holes
- Water resistance figure, or a plain statement that the watch is not rated
If a seller cannot answer these five items in writing, the listing does not carry enough data to judge the watch.