The Elgin National Watch Company Production Records

Each American made Elgin movement (the "movement" is the "watch works" inside the case) has a serial number, and Elgin left behind pretty good records of these. 

This online database for Elgin Watch Company products can tell you a great deal of information about your watch, based on the serial number of the movement, especially for older pocketwatches. The Elgin company is gone, but it left us with factory records of the ranges of the serial numbers engraved on their movements, and the attributes of the movements included in those production runs. The information includes the year, size, grade, jeweling, finish and more. Look up Elgin watch serial numbers here.The original information in the old records was loaded with exceptions, caveats and out right errors. I make corrections to the data available today online almost every month based on actual watches. There's a lot to do! For a rough idea, there's also a simpler table of the serial numbers and production years here, which shows the approximate year only. This information is much less refined, but will yield a good ballpark answer.