Seiko SPB331J1: specifications and what to check

A coated case is a one-way street

Seiko specifies the case as stainless steel with hard coating, and that single phrase should change how you inspect, how you describe and how you price this watch.

Bare steel is forgiving. A scratch can be refinished, a scuffed lug can be brought back, and a competent watchmaker can make a tired case look close to new without removing anything a buyer will miss.

A coated case cannot. The coating is a thin surface layer, and when it wears through you are looking at the steel underneath — a different colour, in a patch, permanently. Polishing does not restore it; polishing removes more of it.

So look where coatings go first:

• Lug edges and corners, where the case meets the world
• The clasp, which is opened and closed daily and rubs against itself
• The bracelet's link edges, for the same reason
• The case flanks at the widest point

Any breakthrough is a permanent condition issue. Photograph it, describe it, and price it in. A buyer who receives a coated watch with a bright steel patch you did not mention will treat it as concealment, because from their side it looks exactly like concealment.

The serial number is part of the product

The case back is see-through and screwed, and Seiko engraves LIMITED EDITION and a serial number on it. The edition is 2,000 pieces.

On a numbered watch, that engraving is not a detail — it is part of what the buyer is acquiring, and serious buyers ask for the number before they commit.

Photograph it clearly, straight on, in focus, at the buy. Record it in your inventory. Quote it in the listing. And check it is legible and undamaged: a serial that has been polished faint on a numbered edition is a genuine deduction.

While the back is in front of you, use the window. Look at the movement under a loupe for oil migration, burred screw heads and rotor rub.

Eighty-three grams, eleven millimetres

Published: 39.3 mm in diameter, 11.1 mm thick, 47.2 mm lug to lug, on a bracelet 195.0 mm long, total weight 83.0 g.

Every one of those numbers points the same way. This is a small, thin, light watch on a short bracelet, and it is a very different object from the 168-gram diver in the same catalogue.

Two of them belong in the listing without being asked for. 11.1 mm is what determines whether it goes under a cuff, and 47.2 mm lug to lug — which is actually longer than the 40 mm diver's 46.4 — is what determines whether it fits a wrist. Diameter alone would have told the buyer neither.

The 195 mm bracelet raises the standard question: where are the removed links? On a limited edition, sourcing replacements is harder than on a standard reference, so a short bracelet is worth more of a deduction here than it would be elsewhere.

Movement and rating

The 6R35: 24 jewels, approximately 70 hours, +25 to −15 seconds per day, stop seconds and a date.

That is the same calibre as the SPB317 diver — a useful reminder that the movement tells you very little about the watch around it.

Water resistance is 10 bar. Describe it that way. It is a perfectly good everyday rating and it is not a swimming specification, and there is no reason to reach for the language Seiko uses on its dive watches.

The crystal here is flat sapphire with anti-reflective coating on the inner surface — not the curved sapphire fitted to the Prospex references. Worth knowing before anyone orders a replacement.