Gruen Movement and Case Serial Numbers

NEW!   Updated Nov 30, 2016
Wristwatch Case Serial Numbers
Cases made by Gruen, Keystone, and Star
Decoding Gruen case and movement serial numbers has been a multi-year, team effort. I want to thank Barry "Bazzab" Cooper, Bernd "Barney" Riemann, and Cathy Orbista for their significant contributions to the project.

While not completed, there has been significant progress this year for Gruen, Keystone, and Star manufactured cases. Prior to this table there was only Wadsworth serial numbers that had been decoded.  There are more prefixes and ranges yet to be completed for these and other case manufactures.  And there are more movement serial numbers to be decoded as well.  All in good time.... this is a great start however!

This table has 3 manufacturers listed - Gruen, Keystone, and Star.  They way it is read is you read the first 2 or 3 characters from the CASE SERIAL NUMBER and locate them in the corresponding columns.  You can then read the year the case was manufactured using the dates going down the table.

+----------+----------+----------+-------------------+----------+-----------------+----------+
|          | Gruen    | Gruen    | Gruen             | Gruen    | Keystone        | Star     |
| Year     | Prefix 4 | Prefix 5 | Prefix B          | Prefix C | Prefix K        | Prefix O |
+----------+----------+----------+-------------------+----------+-----------------+----------+
|          |          |          |                   |          |                 |          |
| 1940     |          |          |                   |          |                 |          |
| 1941     | 42 43    |          |                   |          |                 |          |
| 1942     | 44 45    |          |                   |          |                 |          |
| 1943     |          |          |                   |          |                 |          |
| 1944     | 46       |          |                   |          |                 |          |
| 1945     | 47       | 56       |                   |          |                 |          |
| 1946     |          | 59       |                   |          |                 |          |
| 1947     |          |          | B2 B3 B4 B5 B6 B7 |          | K10 K11 K12 K13 | O36      |
| 1948     |          |          | B8                |          | K14 K15         |          |
| 1949     |          |          | B9                | C1       | K16             |          |
| 1950     |          |          |                   | C2       |                 | O45-O53  |
| 1951     |          |          |                   | C4       |                 | O58      |
| 1952     |          |          |                   | C5       |                 |          |
| 1953     |          |          |                   | C7       |                 |          |
| 1954     |          |          |                   | C9       |                 |          |
+----------+----------+----------+-------------------+----------+-----------------+----------+

Movement Serial Numbers

Cal%20405%202013-12-26.html

Updated Dec 26, 2013

Gruen Movement Dating using Movement Serial Number

For the 405 Family of Movements

405, 406, 410, 411, 420, 421, 425, 426

The serial number encryption for Gruen movements in the 405 410 420 & 425 families has been cracked... or so is hoped. Every few months brings new refinements.

This table is for the 405, 420 and 425 based calibers and is a guide for determining the approximate date of manufacture of a movement.  It’s most applicable for calibers 405, 406, 410, 411, 420, 421, 425, 426 and their sweep-seconds versions. 

The latest refinement includes extending the table backwards in time to 1939.
 

To use these new 1939 and 1940/41 dates, the movement needs to have these two characteristics:

1.
 There is nothing written on the ratchet wheel
2.
 Veri-Thin is stamped on theTrain Bridge


Serial number Year
-------------------------
700,000
         1939
900,000
         1940/1


This table is the same as the previously published one in
 2012, 
but has been refined to be more accurate.

For 405 406 410 411 420 421 425 426 calibers and their sweep versions

Serial number Year
-------------------------
010,000       1941*
400,000       1942**
750,000       1943/4+
900,000       1945
A
 100,000     1945
1A 10,000     1945
1B
 10,000     1945/6
1C
 10,000     1946
A
 900,000     1946
B
 100,000     1947
B
 900,000     1948
C
 100,000     1949
C
 
900,000     1952

Footnotes - To simply use the tables, you need not concern yourself with these notes.

If you want to really get into the details of that data derivation then dig in!

*Some low-numbers after the reset continue to havepre-resetmarkings
**Coincident with lowest 405ss/406ss movements 
+ Coincident with the lowest 420/421 (sub and SS calibers) observed.
Coincident with the highest 405/406 caliber and lowest of the 425/426 calibers

A few caveats when dating your watch, specifically when using movement serial numbers: 

1 - That movements could have been made years prior to its incorporation into your watch. If this is true for your watch, the movement will render a date earlier than your watch’s age
2
 - Movement swapping is a common repair method, so be aware your watch may not contain its original movement and thus the date will be incorrect

Copyright 2012, 2013 M. Barnett & J. Wood

Location of serial number on Gruen Watch movement

Wadsworth Case Serial Numbers

In addition to the movement serial numbers, cases made by Wadsworth can sometimes be used to date a watch.  Check out the Wadsworth Case Serial numbers page in the Reference Section.

Download a PDF version of the table movement serial number table