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Fortis · Cosmonaut

Strap for Spacematic Counterrotation

Takes 20 mm straps. Verified lug width, the straps that fit this model, and answers to the most common questions.

Lug width

20mm
20 mm strapsAll Fortis
Lug width20 mm
Case diameter40 mm
Reference623.10.51
FamilyCosmonaut
MovementETA caliber 2836-2 hours, minutes, seconds | date, day
CaseStainless Steel

Spring bar

Swiss Standard universal spring bars

The right category for this model · 316L steel, all lug widths 13–26 mm

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Straps for the Cosmonaut 623.10.51

Selected for the 20 mm lug width · workshop selection

All 20 mm straps →

The Cosmonaut 623.10.51, seen from the bench

The Fortis Cosmonaut ref. 623.10.51 is equipped with the automatic ETA 2836-2 caliber with hour, minute, second, date and day of the week indication. The watch features 20mm lugs and is part of the Swiss brand's aerospace collection. The movement provides the essential functions for professional use in the aeronautical field.

Frequently asked questions

What is the exact lug width of the Fortis Cosmonaut 623.10.51?

The lug width of the Fortis Cosmonaut 623.10.51 is precisely 20 millimetres, measured between the inner edges of the suspension pins. This dimension is exacting and permits the fitting of any strap designed for 20mm lugs, whether bespoke or mass-produced.

Which strap is most suitable for the Cosmonaut 623.10.51?

For the Cosmonaut 623.10.51, we recommend vegetable-tanned leather straps in dark tones (brown, black, navy) to emphasise the tool-watch character, or NATO straps in Cordura nylon to reflect the functional heritage of the pilot's chronograph. FKM rubber is optimal for aquatic or extreme environments.

How does one change the strap on the Fortis Cosmonaut 623.10.51?

Position the watch horizontally with the case facing upwards. Use a strap-change tool or fine needle to gently push the pin from opposite sides, allowing the old strap to slide free. Align the new strap with the pin apertures and insert the pin until it seats fully on both sides.

Is an aftermarket strap compatible with the Cosmonaut 623.10.51?

Yes, provided the strap is engineered for 20mm lug width. Any bespoke or mass-produced strap with this specification is mechanically compatible with the Cosmonaut 623.10.51, delivering identical security and reliability.

What is the expected lifespan of a leather strap on the Cosmonaut 623.10.51?

A vegetable-tanned leather strap for the Cosmonaut 623.10.51 typically lasts 3–5 years with daily wear, depending on ambient humidity, perspiration and frequency of use. Periodic conditioning with appropriate creams extends longevity considerably. Synthetic materials such as rubber and nylon offer extended lifespans with minimal maintenance.

The next step

The right strap isn't chosen. It is built, together.

From the atelier appointment to delivery, every step is made for those who know that things done well take time. Rare leathers, one piece at a time, eighteen months of warranty.

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Casati Milano · Via XX Settembre 15, Milan · by appointment only