There isn't a lot of info on these old guys. My old Washy flat top can only be dated to the 1920's, and that's not by serial number but by a detail here and a detail here that was done at a certain point in production. All in all it all boils down to sometime in the 20's.
Usually, it takes a hands on look by someone with some sort of professional experience--collector or investor-- of old instruments to get even an estimate of the birth date.
A letter typed on a keyboard is a note stolen from a fretboard.