Seiko SPB379J1: specifications and what to check

Twenty bar is not a diver's rating

Seiko is careful about this and a dealer should be too.

The water resistance here is published as 20 bar. Not "200m diver's" — just the pressure figure. Elsewhere in the same Prospex family, the SPB317 is published as 200m / 660ft diver's, and the SPB453 as 300m / 1000ft diver's.

Twenty bar is a substantial everyday rating. It is not the same claim, and this watch comes on cow leather, which settles the question of what it was built for.

So describe it as Seiko does. "20 bar" is accurate and defensible. Calling a Prospex on a leather strap a 200-metre diver because the pressure figure converts is the kind of sloppiness that ends in a soaked watch and a conversation you cannot win.

The compass ring, and the crown that drives it

The distinguishing feature is a rotating compass inner ring — a bearing ring inside the crystal rather than a bezel on the outside, turned by a second crown on the case.

Three things to check, and none of them apply to a normal bezel.

It must turn from the crown. You cannot grip an inner ring. If the crown turns and the ring does not, the connection has failed and the feature is gone.

It must hold. Set it, let go, and shake the watch gently. A ring that drifts under its own weight cannot be used for a bearing.

Its crown must screw down. This is a second penetration of a 20-bar case, and it is the one an owner forgets. Confirm the thread engages cleanly and that the crown sits flush.

Demonstrate it to the buyer as well. Plenty of people buy an Alpinist without ever discovering that the inner ring is a working compass scale rather than decoration.

A Prospex you can see into

The case back is specified as see-through and screw — unusual in a range built largely on solid, sealed backs.

Use it. Wind the watch and look at the rotor, the bridges and the jewels under a loupe. Fingerprints, slipped screwdriver marks and oil creeping from a jewel are all visible through a display back and invisible through a steel one.

Check the sapphire in the back for chips at its edge while you are there. A display back is a second piece of glass in the case, facing the wrist, and it takes knocks nobody thinks about.

The 6R54, and what to test over a day

The movement is the 6R54: automatic with manual winding, 24 jewels, approximately 72 hours, precision published as +25 to −15 seconds per day, with a 24-hour hand for a second time zone.

The reserve figure gives you a three-day test, and the GMT hand gives you a one-day test worth running alongside it.

Put the 24-hour hand eight hours away from the local reading, then leave the watch alone. A day later the gap between the two should still be exactly eight hours. A second time zone that has drifted against local time is a movement fault, and it is completely invisible in a five-minute inspection.

The case is stainless steel with a super hard coating, which is worth noting: a coated case cannot be polished back the way bare steel can, so wear through to the substrate is permanent.

Published dimensions: 39.5 mm across and 13.6 mm tall, spanning 46.4 mm between the lug tips, with 20 mm between the lugs themselves. Magnetic resistance 4,800 A/m. Seiko does not publish a weight for this reference, so if a buyer asks, weigh it and say that you weighed it.