Runs from the 6159-7001 (1968, a 300m hi-beat professional diver) to the epochal 6159-7010 "Grandfather Tuna" of 1975 — the world's first diver with a titanium monocoque case and Seiko's first 600m professional rating, developed by Ikuo Tokunaga's team in response to a 1968 letter from a saturation diver in Kure City whose watches failed at depth. The Tuna solved helium ingress not with an escape valve but through robust construction and an L-shaped gasket, adding a ceramic-coated titanium shroud (the "tuna can" shape), an accordion strap, and over 20 patents. Collectors chase the earliest examples lacking the "m" after "600."
