Seiko SPB453 vs Alpinist SPB317

300m rather than 200m, in a 40mm case that is only 46.4mm lug-to-lug and 13mm thick. For a watch rated to thirty atmospheres, that is remarkably contained — most 300m watches are noticeably larger.

The 6R55 runs 72 hours with a published +25/-15 seconds a day, and Seiko publishes $296 to overhaul it. Nothing about the cost of the next thirty years is a guess.

It is the more expensive watch and it looks like a dive watch, which is the point but also the limit — the bezel and the case read as purposeful rather than quiet, and it is the less versatile of the two under a cuff.

300m and a 46.4mm lug-to-lug in the same watch is unusual. The extender in the clasp is the detail that says Seiko still expects this one to be dived, not just worn.

The Alpinist is a field watch with a diver's water rating: 200m, sapphire, an internal compass bezel, and the capability without the dive-watch presence. 41mm across, 46.9mm lug-to-lug, 12.3mm thick.

The 6R35 runs 70 hours with the same published accuracy tolerance and the same $296 overhaul — you give up two hours of reserve and 100m of rating, and nothing else.

It is the better single watch and the worse specialist. If you actually dive, the rotating timing bezel on the SPB453 is the tool you want; the Alpinist's internal bezel is not the same thing.

This is where Seiko starts publishing an accuracy figure (+25/-15 seconds a day) and fits sapphire. A 6R35 overhaul is $296 against $176 for a 4R36 — a real difference, and one you can plan for.