The “Clamshell” case, protected under Swiss patent No. 189190 (1936–37), was developed by Schmitz Frères and later licensed to Gallet.
Its design used four screws at the lugs to clamp the case back and crystal together, creating one of the first truly waterproof chronograph cases.
This construction provided greater protection against moisture and dust than most watches of the 1930s.
While Gallet became the best-known user of the design, Breitling also produced early chronographs housed in Clamshell cases.
These rare Breitling examples often feature “Brev. 189190” engraved inside the case back, along with “Waterproof / Acier Staybrite.”

Today, Breitling Clamshell chronographs are prized by collectors for their rarity, engineering ingenuity, and historical importance in the evolution of waterproof chronographs.