For the most part, case serial numbers only server to confuse people about what kind of watch they have.
New models of watches may be released before or after these general dates.
The jeweler would then put the movement and the case together.
With only a few exceptions, the watch companies and watch case companies were completely unrelated business.
The table below with the serial numbers and years has be compiled from many sources and I believe it to be reasonably accurate, However, no table like this can be completely accurate because Elgin didn't produce watches in sequential order.
It was very common for groups of watches to be set aside and finished months, years, or sometimes as much as a decade later.
It is very important to not use the serial number on the case because that is completely unrelated to the movement serial number.