The term waterproof and its English translation had a very different technical and commercial weight throughout the twentieth century compared to what water resistant carries in today's language. This is not merely a semantic question: it's a story of patents, regulations, and marketing that anyone buying or selling vintage watches must understand.
The regulatory turning point came in 1970, when Seiko modified cases and dials to comply with American legislation, which forbids the use of waterproof as an absolute promise impossible to guarantee. From that moment on, water resistant became the international standard. Pre-1970 Seiko cases bearing the original wording are systematically more sought after by collectors, not out of nostalgia but because that marking identifies a precise generation of production.
In British military circles, the acronym W.W.W. — sources attest to both Wrist Watch Waterproof and Watch Wristlet Waterproof, both circulating in period documents — appears engraved on the caseback of watches commissioned by the Ministry of Defence in 1945 as a replacement for the earlier A.T.P. These pieces, twelve references produced by as many Swiss manufacturers, are today collectively known as the Dirty Dozen. On the caseback, the acronym is often accompanied by three impressions of the Broad Arrow, the Pheon, the symbol of British Crown property.
From a constructive standpoint, water resistance is achieved through very different systems. The screw-down crown compresses a gasket against the case, creating a positive mechanical seal: this is the solution adopted by diving watches. However, alternative approaches exist: the Swiss patent 189190 by Schmitz Frères (1939) for the Gallet Multichron uses a clamshell case in which a bucket-style back slides inside the front section, with four screws hidden in the lug feet compressing the two sections, leaving no screws visible on the caseback. The Bulova capsule-type casing system, Fachon patent of 1959, instead forms a unitary capsule of movement, dial, and crystal that fits into an external bezel with integrated lugs: the crystal's grooves align with raised rings on the case, guaranteeing water resistance without external screws. More recent systems, like the triple gaskets on the MING 22.01 GMT crown, achieve high protection without resorting to a screw-down crown.
At the workbench, I still learn something every time I open a case from the Fifties: the original gasket is almost always hardened or missing. A waterproof declaration on the dial and actual water resistance are two separate things. Before immersing—even in tap water—a gasket overhaul is the only concrete guarantee, regardless of what's written on the dial.