Seiko Model Number Decoder

Read left to right. The four characters before the hyphen are the caliber and the four after it describe the case and dial. Some Seiko movements sold as standalone units through the company’s Time Module division are written without a case code — for example NH35A or NH36A — because they are the bare movement that microbrands and modders drop into their own cases. In those, the caliber is the whole thing you see.

Alongside the caliber-case number, every modern Seiko has a retail reference such as SRPD55K1, SNK809 or SKX007J1. This is the code you shop by. The leading letters flag the collection or family, and the digits pick a specific model within it. It is tempting to expect a tidy position-by-position key, but there isn’t one you can rely on across the whole catalogue — Seiko treats these as marketing codes, not a public specification format. Decode what you safely can, and read the caliber-case number for anything that actually matters to the mechanics.

The one part of an SKU that carries a widely-followed convention is the trailing letter. A J commonly indicates a Japan-made or Japan-assembled variant, frequently with a “Made in Japan” dial, while a K commonly indicates a version assembled outside Japan. In practice Seiko applies its quality control across facilities, so for most models the difference is about provenance and a small resale premium rather than build quality. Treat J and K as strong conventions, not guarantees, and always confirm against the specific reference.

These two features are the clearest dividing line between Seiko’s entry and mid-tier automatics, and the decoder reports both. Hacking means the seconds hand stops when you pull the crown out to set the time, so you can synchronise the watch to the second. The 7S26 does not hack; the 4R and 6R families do. Hand-winding means you can wind the mainspring manually via the crown without shaking the watch on your wrist — handy for starting a watch that has stopped. Again the 7S lacks it while the 4R and 6R have it. If a listing claims a 7S26 hacks and hand-winds, either the movement has been swapped or the description is wrong.

The NH35 and NH36 are worth calling out: they are the same movements as the 4R35 and 4R36, produced by Seiko’s Time Module (TMI) arm and sold to other watch companies and to the modding community. If your caseback reads NH35A with no case code, you are almost certainly looking at a microbrand or custom build using a Seiko movement.

None of these alone proves a fake, but each is a thread worth pulling. Cross-reference the caliber, the case code, the SKU letter and the physical watch — and when in doubt, confirm the production date with the serial number lookup.

What is a Seiko model number and where do I find it?

A Seiko caseback carries two numbers. The caliber-case number looks like 7S26-0020 or 6R35-00P0: the first part is the caliber (the movement), and the second identifies the case and dial variant. Separately, the retail SKU such as SRPD55K1 or SKX007J1 is the marketing reference on the box, warranty card and hangtag. Both are on the caseback, with the SKU-related markings near the model code above the serial.

How do I tell the caliber from a Seiko model number?

The caliber is the first block before the hyphen. In 7S26-0020 it is 7S26; in 4R36-01J0 it is 4R36; in 6R35-00P0 it is 6R35. Standalone Time Module movements are written as NH35A or NH36A. Once you know the prefix you know the family, and therefore the movement type, hacking and hand-winding, and typical reserve.

What is the difference between the Seiko 7S26 and 4R36?

Both are entry-level automatics at 21,600 vph with roughly a 40–41 hour reserve. The difference is features: the older 7S26 cannot be hand-wound and does not hack, so the seconds keep running when you set the time. The 4R36 adds hand-winding, hacking and a day-date (the 4R35 is date-only). Real-world accuracy is broadly similar.

Can a Seiko SKU like SRPD55K1 be fully decoded?

Not by a fixed formula. SKUs are marketing codes: the leading letters flag the collection and the digits pick a model, but there is no public position-by-position key. The most useful clue is the trailing letter — a J commonly means a Japan-made or Japan-assembled variant and a K a version assembled elsewhere. Treat these as conventions and confirm the caliber from the caliber-case number.

What do the letters J and K mean on a Seiko?

On many modern SKUs the trailing J or K signals assembly origin. A J variant is generally Japan-made or Japan-assembled and often carries a “Made in Japan” dial, which can command a small premium in the Japanese market; a K variant is generally assembled outside Japan. Seiko applies quality control across facilities, so for most models the real-world quality gap is minimal. It is a convention, not an absolute rule.

How do I know if my Seiko is automatic, quartz, solar or Spring Drive?

The caliber prefix tells you. 7S, 4R, 6R, 8L, 9S and NH are mechanical automatics. A V prefix (like V147) is solar quartz, charged by light rather than a disposable battery. 7A and 7T are battery quartz chronographs. The 9R (and 5R) families are Spring Drive — a mainspring regulated by a quartz oscillator with a gliding seconds hand — while 9F is high-accuracy Grand Seiko quartz.