The term waterproof has had a precise regulatory history: until 1970 it was stamped on dials and cases as an absolute claim, after which the American Federal Trade Commission forced manufacturers to adopt water resistant, a more cautious and technically honest term. On pre-1970 Seiko examples this transition is visible and documentable piece by piece; for many collectors the specimen with the waterproof marking on the dial is worth something more, not because it is technically superior, but because it tells of an era when marketing was more unscrupulous than physics.
The British military designation W.W.W. — Wristlet Watch Waterproof, according to some sources, or Wrist Watch Waterproof according to others — appears engraved on the caseback of the so-called Dirty Dozen, a family of twelve references commissioned by the British Ministry of Defence in 1945 to replace the A.T.P. The inscription was accompanied by three impressions of the Broad Arrow, the symbol of Crown ownership. The same acronym appears on the crown of the King Seiko 4420-9990 as a registered trademark Seiko W, confirming how laden that term was with commercial value in the post-war period.
From a construction standpoint, manufacturers solved the waterproofing problem in different ways. Swiss patent 189190 by Schmitz Frères, filed in 1939 and applied to the Gallet Multichron Waterproof, exploits a clamshell case: the bucket-shaped back slides inside the front piece and four screws hidden in the lug feet compress the two sections when tightened, with no visible fasteners. The Bulova capsule-type casing patented by Emil Fachon in 1959 goes in the opposite direction: movement, dial and crystal form a unitary capsule that inserts into a thin outer bezel, with grooves on the crystal that align with raised rings on the case. The screw-down crown is the solution that became standard on diving instruments: the thread compresses a gasket against the case, sealing the interior. Rolex's Twinlock adds a second sealing plane inside the same stem. Some modern references, such as the MING 22.01 GMT, forgo the screw and instead use a crown with triple gaskets, a solution that preserves ease of use at the expense of maximum water resistance.
From my workbench I note that rubber gaskets age and compress: a watch that was certified waterproof in 1965 is no longer so today without servicing. Before any water testing on a vintage piece, a gasket revision is always worthwhile.