There are several ways to get an idea of when your watch was made.
Some companies (as with the Gruen Watch Company), even lost rocords to fire or other natural disaster. I am sure they had no idea that their timepieces would someday be collected.
The code consists of one letter and one number, eg. They used the letter M to signify the decade 1960 to the end of 1969, and the letter N to indicate 1970 to the end of 1979.
The digit represents the actual year in that decade. It is possible that you may have a watch in which the date code on the movement is different to the date code on the case.
There are no number tables that I am aware of to cross reference the date of manufacture with the serial number.
The serical numbers were simply for identification purposes only.
They were the first watch company to market watches specifically to women, and the first company to open a school for disabled veterans that provided them with care, employment opportunities, and a marketable skill after the war (more below).