Seiko case-back markings and serial numbers

Serial number

Seikos can have an 8-digit serial number, 7-digit serial number, 6-digit serial number, no case serial number but with movement serial number, no case serial number or movement serial number, or a new non-date serial number system. Most serial numbers are year-month, but some earlier ones from the 1950s and 1960s are month-year (do month-year 6105s go into the 1970s?)

8-digit serial numbers (1940s?)

7-digit serial numbers

6-digit serial numbers

No case serial numbers but with movement serial number

No case serial number and no movement serial number:

Some stainless steel Seiko Uniques and Seiko Marvels.

Non-date serial number system - new SEIKO 5 SPORTS.

Month-year serial numbers:

Found in some but not all:

Seiko Unique (Daini Seikosha), Seiko Marvel (Suwa Seikosha), Seiko Lord Marvels with Marvel-base mvovement (Suwa Seikosha), Seiko Cronos, 6105s (Suwa Seikosha).

Source: https://www.tapatalk.com/groups/seikoholics/think-you-know-seiko-serial-s-t1172.html

Case model codes:

Earliest system? 4 digits

Dial diameter in lignes and fractions of lignes system - 5 numerical digits, with the first numerical digit beginning in 1. "J" pre-fix to the numerical digit indicates an extra fraction of a ligne, probably 1/2 a ligne. Runs from 8xxx? to 17xxx.

Early movement and case model number code system - using first 2 digits of 3-digit movement number, plus a 3-digit or 4-digit case code. ~1963-1964.

New movement and case model number system.

Case material codes:

https://www.plus9time.com/seiko-case-codes/

Case manufacturer codes:

A

B

C

F

G

J - Tenryu Industry

L

M

N

P

Q

S

U

X

TL