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The Elgin and Hamilton were dated by serial number, while the Gruen was dated by an inscription on the back. In the 1930's there was a style change from highly engraved cases to smoother cleaner lines.The "New Haven" was dated by looking at the style of the watch. Notice that the watches from the early 1930's still have engraved cases.
The very early Tissot watches had the serial number engraved on the back site of the dial.
These numbers are stamped on the back (outside) of the case. The code was usually stamped on the back of the watch case.
If there is no two symbols, it could be that your watch was made before 1948. For exabple, If you have a Bulova watch with the stamp M3 then it was made in 1963.
The two numbers serves the same pourposes as explained in the Omega chapter above-written.
Though in the periode dating from 1958 to 1974 the reference number on Certina watch carried an encrypted message giving away, which calibre the watch was equipped with, which material the case was made from and which watch model you are dealing with.
Note the thick hands, engraved patterns and bold design of all of these watches.