In general, the more interesting the watch is, the less accurate the serial number will date it.
The serial numbers were allocated in "runs" or blocks of identical grade watches.
Starting in the late 1930s, Elgin added a letter prefix to the serial numbers of some watches and around this time, Elgin also started to mark the grade number on some of the movements in addition to the serial number.
Similarly, high end watches were often slow sellers and there is evidence in the Elgin Master Records that Elgin was still finishing and selling watches over a decade after the serial number would normally imply.
Starting in the late 1930s, Elgin added a letter prefix to the serial numbers on some watches.
Elgin organized the serial numbers into "runs" of identical grade watches.