Seiko introduced the outer protective shroud in 1975 for professional saturation divers who required high shock resistance at extreme depths. Released in 2015 under the Prospex line, the SRP637K1 adapts this 1975 shrouded case design for 200m recreational diving by bolting a cylindrical stainless steel guard around a 48mm steel case.
Known among collectors as the Baby Tuna, this reference combines the protective shroud with the dial layout and 120-click unidirectional bezel of the 2nd-generation Seiko Monster. The watch runs on the automatic Caliber 4R36, which operates at 21,600 vph with a 41-hour power reserve, manual winding, and a hacking seconds mechanism.
Established in 1881 by Kintarō Hattori, Seiko is defined by its principle of complete vertical integration, manufacturing every single component of its timepieces in-house. They were the primary catalyst of the "Quartz Crisis" with the commercial release of the 1969 Astron—the world’s first quartz watch—the brand simultaneously advanced mechanical horology, developing one of the first automatic chronographs and pioneering high-frequency movements to challenge Swiss standards.
Seiko's relentless innovation continues with proprietary technologies like Spring Drive, which blends mechanical power with quartz precision. This technical mastery underpins Seiko's legacy in creating archetypal and exceptionally durable tool watches, particularly in the domain of professional diving.
Overall Grade: 7,5 / 10 | Good Condition, Medium Wear Production: Undated Contents: Full-set Genre: Dive Watch
Size: 48mm Thickness: approx. 14mm Lug Width: approx. 22mm
Case: Stainless steel shrouded case, brushed finish, medium wear Bezel: Unidirectional dive bezel, brushed finish, medium wear Dial: Matte black dial, lumed applied markers, day-date display Hands: White 'Prospex Samurai' hands
Crown: Stainless steel, screw-down crown at 4, medium wear Crystal: Hardlex mineral crystal, medium wear Bracelet: Stainless steel bracelet, additional nylon strap, medium wear Caseback: Stainless steel, solid caseback, screw-down, medium wear Movement: Automatic Caliber: 4R36