The fourth collaboration between Seiko and Rowing Blazers, released in 2026, returns to a concept introduced in the partnership’s first chapter: the Rally Diver, a diver’s watch whose rotating bezel insert carries a checkered racing flag pattern rather than the conventional elapsed-time scale. What is new this time is a deliberate split in case sizing and colorway assignment. The green dial version is housed in a 38mm case; the blue dial version occupies a 42.5mm case. Each is a limited edition of 2,500 pieces, for a combined run of 5,000. The two references are the SRPM21 (38mm) and the SRPM19 (42.5mm).
That pairing—smaller case, green dial; larger case, blue dial—is not arbitrary, but it does create a set of design problems worth examining carefully. When you change case diameter, you are not simply scaling a drawing up or down. You are altering the relationship between every element on the dial and every surface on the case.
What the Size Difference Actually Means for Proportion
The 42.5mm case measures 13.4mm thick with a lug-to-lug distance of 46mm. The 38mm case is 12.1mm thick with a lug-to-lug of 44.2mm. The lug-to-lug reduction of 1.8mm is modest, but the case diameter reduction of 4mm is more consequential for how the dial reads. A bezel that works proportionally at 42.5mm—wide enough to carry the checkered insert pattern with visual clarity—must do the same job on a case that is meaningfully narrower. The printed anodized aluminum insert has less circumference to work with, which compresses the pattern and tightens the spacing between the flag segments.
This is where colorway assignment becomes a design variable rather than a purely aesthetic one. Green, as a mid-tone color with relatively high visual weight, tends to hold its presence on a smaller dial better than a cooler, lighter hue might. Blue, particularly at the saturated end of the spectrum, can read as expansive—it benefits from the additional real estate of the 42.5mm case. Whether this was the explicit reasoning behind the assignment is not stated, but the pairing is coherent from a visual logic standpoint.
Dial Density and the Day-Date Complication
Both watches run the Seiko caliber 4R36, an automatic movement operating at 3Hz with a 41 hours power reserve. The 4R36 includes a day and date display, which means both dials carry that complication regardless of case size. On the 42.5mm, a day-date window sits within a dial that has room to absorb it without crowding the other elements. On the 38mm, the same complication occupies a proportionally larger share of the dial surface. This is a recurring tension in smaller-cased watches that retain complications designed for larger formats: the indices, hands, and date aperture compete for space in a way that does not arise at 42.5mm.
The dial construction in both cases relies on applied indices, which is consistent with the Seiko 5 Sports platform. 
Bezel and Crystal
The unidirectional rotating bezel on both references uses a printed anodized aluminum insert—the checkered pattern that defines the Rally Diver identity. Aluminum inserts are standard across the Seiko 5 Sports range and are appropriate here given the tool-watch register the collaboration occupies. The crystal on both is Seiko’s Hardlex mineral crystal, which sits above the dial and on the caseback and carries a printed graphic of a dripping faucet—a reference to Rowing Blazers’ internal “Bathroom Club” iconography.
Bracelet and Strap Geometry
Each watch ships with a matching stainless steel bracelet and a contrasting NATO-style strap. The strap widths follow the case sizes: the 42.5mm blue watch uses a 22mm strap; the 38mm green watch uses a 20mm strap. Notably, the strap colors are inverted relative to the dials—the blue watch receives a green strap, and the green watch receives a blue strap. This cross-referencing between the two pieces is a considered detail that reinforces the idea of the two watches as a paired set rather than independent releases. The steel bracelets are non-tapering, which is a characteristic of the Seiko 5 Sports platform at this level.
Display Caseback
Both references feature display casebacks, allowing the 4R36 movement to be viewed from the rear. This is a practical addition that aligns with the enthusiast-facing intent of the collaboration—it acknowledges that the person wearing the watch has an interest in the mechanism, not just the dial.
Both cases are stainless steel with 100 meters of water resistance, and the crown is positioned at 4 o’clock with crown protection on both references. The overall construction is consistent across the two sizes; the meaningful differences are dimensional and chromatic. With all 5,000 pieces sold through primary channels at launch, the 2026 Rally Diver will be found primarily on the secondary market going forward.
