After granting its official approval to the first Grands Fonds 300 in 1960, the French Navy explained to the designers that the crown at 3 o'clock posed a problem as it was too vulnerable during underwater manipulations. It asked ZRC to “find a solution” to protect the crown without impeding wrist movement, and above all, to ensure it could not open under water. The team set to work and developed the famous crown at 6 o'clock, connected to the case using an exclusive system consisting of a canon cut from a single block, making the Grands Fonds 300 the first and only watch to be equipped with a 100% monobloc case. Furthermore, the case is machined in a particular type of steel reinforced with molybdenum that is completely antimagnetic (an absolute necessity for Mine Clearance Divers' Group).