Back pocket dating
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.
These numbers are stamped on the back (outside) of the case. The code was usually stamped on the back of the watch case.As you can see from some of these examples, bold and fancy was back in style! Hands got a little wider and came to a sharper point.Dials were larger while the cases were made to be less of a focal point...Other sources attribute the expansion of the original red–blue system into today’s code to marketing efforts around 1971 by the San Francisco department store for erotic merchandise, The Trading Post, promoting handkerchiefs by printing cards listing the meanings of various colors. S Leather in San Francisco, claimed that he created the first hanky code with his business partners at Leather 'n' Things in 1972, when their bandana supplier inadvertently doubled their order and the expanded code would help them sell the extra colors they had received.This table is drawn from Larry Townsend's The Leatherman's Handbook II (the 1983 second edition; the 1972 first edition did not include this list) and is generally considered authoritative.