The Sinn U1, introduced in 2005 according to the CSV, is a diver’s watch with a case made of U-boat steel, a German submarine-grade steel that is tougher and more corrosion-resistant than standard stainless steel. It has a 44 mm case, a water resistance of 1,000 metres and a rotating bezel, and runs a Swiss automatic movement. I could not confirm the introduction year or movement from an official source.

Sinn pioneered the use of submarine steel in watch cases and the U1 is the flagship of the idea. The watch is built to professional diving standards and has been offered in several finishes and with a bracelet or a strap, including a patina-dial version.

Owners and reviewers value it for its toughness and for the value it offers for a watch with this level of specification. The U1 remains in production as part of Sinn’s diving range.