Nomos Glashütte Tetra / Open Case Back Ref. 406

Reference 406 is the sapphire-back version of the NOMOS Tetra, with a 29.5mm square stainless steel case, white silver-plated dial, tempered blue hands, and manual winding. Like the closed-back version, this long-running Tetra reference has gone through several hand-wound movement iterations over time, including 1T, 1TS, 1TSP, and later Alpha executions before the current DUW 4001 calibre. The case measures 29.5mm by 29.5mm, with a height of 6.5mm and a lug-to-lug length of 40.5mm. It has a sapphire crystal front and a sapphire crystal case back, with the stainless steel case fixed by four screws. The Tetra case is defined by its square shape, stepped lugs, compact proportions, and flat profile. Current specifications list water resistance at 5 atm, an increase from the original 3 atm rating. The galvanized white silver-plated dial uses the Tetra’s square minute track, printed Arabic numerals, and a recessed small seconds display at 6 o’clock. The tempered blue steel hands contrast with the pale dial surface. The display consists of central hours and minutes with subsidiary seconds. Power currently comes from the NOMOS DUW 4001, a manually wound in-house calibre measuring 10 ½ lignes in diameter and 2.6mm in height. It has 17 jewels and a power reserve of up to 53 hours. The movement is visible through the sapphire crystal case back. The watch is supplied on a black Horween Genuine Shell Cordovan strap with an 18mm lug width.