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Curiously, the process for dating the earliest Gruen wristwatches -- from about 1915 to 1935 or so -- is probably the easiest because of the limited number of movements that Gruen had available, and the resulting relatively small number of specific models.
The wonderful Gruen Guild Book that the company put out between 1929 and about 1931 (and widely available online) is arguably close to a "Masterbook" of Gruen watches, particularly wristwatches made up to that time period.
If you want to learn more about the application of Style Number Dating to help understand when each of Gruen's movements went into production and stopped being used in production, check out the analysis of in the Reference Section.
It's a visual view of each movement's use of Style Numbers. It shows how many Style Numbers were associated with Caliber 311 movements and what years those Style Numbers represent.
Download Ver 4.0 of the free Windows program that provides date and other information given a Style Number.
This is the standard labor involved in collecting the basics.
It is from raw data such as this that patterns begun to emerge if you collect enough of it.
These and other brands have volumes of material and definitive movement serial number listings that allow for precise dating down to the year, and in some instances even the MONTH, in which a given specimen was manufactured.
Not so for Gruen, being the "orphan child" without a birth certificate because of a rather complicated method of serializing their movements, the records for which have been long lost, and most likely destroyed.