While I'm not an expert, I believe the information on this page is correct. Please send suggestions and corrections to the webmaster. The depressed crown will also release case back cover.
"The serial number is on the movement not the case".Note that we're using the serial number from the watch movement, not from the watch case.Looking at the table of Waltham serial numbers (see example below), you can see that number 20,900,000 was made in 1917 and 21,800,000 was made in 1918 (marked in red in the table below). Your movement serial number may not be in exactly the same location as the one in the photo, but you are looking for the serial number that is on the watch mechanism itself... This is an example using the Waltham serial number table.The Online Database for the Elgin Watch Company can tell you a great deal of information about your watch, based on just the serial number off the movement.(The "movement" is also sometimes called the "watch works".) Important!