[1][2] Introduced before World War II as part of Gallet's MultiChron line of military and professional timepieces, Clamshell watches are distinguished by four screws on the back of the case, positioned at the base of the lugs where the band or bracelet attaches.
The Clamshell's 15-year history began in 1936 when Gallet purchased the rights to a patent for the "waterproof" watch case design (Brevet N° 189190), invented by the Swiss firm of Schmitz Frères & Cie in the neighboring city of Grenchen.
Originally engineered by Gallet for the adverse conditions of the battlefield, the Clamshell chronograph derived its nickname from the way that the upper and lower sections of the watch case compressed together over a specially flared crystal, protecting the delicate inner mechanisms from the intrusion of water, dirt, dust, and caustic chemicals.
In spite of the Clamshell's effectiveness at preventing water intrusion to a submerged depth of over 100 meters, it was eventually replaced by more easily serviceable versions with full screw-on case backs and readily available generic crystals.
Due to the effectiveness of the Clamshell's water resistant case at protecting the watch's inner mechanism against the elements, vintage examples occasionally appear on the market and at auction in fully functional condition, often with non-degraded original dials.