Seiko · Prospex Sea and Prospex Land

Seiko SPB317J1 vs Seiko SPB379J1

11 published specifications differ between these two, and 5 are identical. Every figure below is what the manufacturer states for that reference; nothing is inferred from the other watch.

What differs

Specification	Seiko Prospex Diver 200m, 6R35, silicone bandSPB317J1	Seiko Alpinist Mechanical GMT, 6R54, rotating compass inner ringSPB379J1
Case	Stainless steel, 41.0 mm	Stainless steel with super hard coating, 39.5 mm
Diameter	41 mm	39.5 mm
Thickness	12.3 mm	13.6 mm
Lug to lug	46.9 mm	46.4 mm
Water resistance	200m / 660ft diver's	20 bar
Bezel	Unidirectional rotating bezel	Rotating compass inner ring
Case back	Screw case back	See-through and screw case back
Bracelet	Silicone band, 20 mm between the lugs	Cow leather band, 20 mm between the lugs
Calibre	6R35	6R54
Power reserve	Approximately 70 hours	Approximately 72 hours (3 days)
Movement notes	Precision stated as +25 to −15 seconds per day. Stop second hand and date display. Magnetic resistance 4,800 A/m. LumiBrite on hands, indexes and bezel. Total weight 100.0 g.	Precision stated as +25 to −15 seconds per day. 24-hour hand for dual time display, stop second hand function. Magnetic resistance 4,800 A/m. LumiBrite on hands and indexes.

What is identical

Worth knowing as much as the differences: these are the specifications you cannot use to tell the two apart.

Lug width: 20 mm
Crystal: Curved sapphire crystal, anti-reflective coating on inner surface
Crown: Screw-down crown
Movement: Automatic with manual winding
Jewels: 24

Stated for one, not the other

These rows cannot be compared, because one maker publishes the figure and the other does not. An empty cell is a gap in the manufacturer's documentation, not a gap in the watch — and not a licence to fill it in from the other column.

Clasp	not stated	Three-fold clasp with push button release

What the differences cost you

Each note below follows from the published figures in the tables above — what they mean for intake, for a listing, and for the handover. Nothing here estimates a price or a margin.

Different movements to verify and to service

The SPB317J1 runs the 6R35 and the SPB379J1 the 6R54. Two consequences at intake: the service history that is normal for one calibre is not evidence about the other, and the calibre is the check that catches a movement swapped into the wrong case. Confirm the movement matches the reference rather than assuming it from the dial.

What to check on each

SPB317J1

Bend the silicone band fully back on itself at both lug ends and look into the crease. Silicone fails from inside a fold, and a flat inspection will miss cracking that is about to become a failure.
Confirm the calibre is the 6R35 rather than the 6R55 used elsewhere in this family. Both are 24-jewel automatics from the same maker, and they carry different reserves.
Weigh it against the published 100 grams. That is remarkably light for a steel 200-metre diver and is largely the band rather than the case, which matters if a buyer intends to fit a bracelet.

SPB379J1

Turn the inner compass ring through a full revolution using its own crown and confirm it moves smoothly and holds position. It is driven from a second crown, so a seized ring is a crown or pinion problem rather than something you can free by hand.
Note the water resistance is given as 20 bar, not as a diver's rating. This is a Prospex on a leather band and it should not be described in the language Seiko reserves for its dive watches.
Check both crowns screw down. There are two penetrations of the case here and the one for the inner ring is the one owners forget to secure.

Sources

Not affiliated with Seiko. Specifications are read from each manufacturer's own published documentation on the dates shown and may change; the manufacturer's current documentation governs.