Illinois pocket watch dating
There was no standard across the industry and individual watchmakers may change codes over the years.
For the most part, case serial numbers only server to confuse people about what kind of watch they have.
Or, if they were more interested in fashion than the watch's time keeping ability, they would pick out a case they liked and used the money left over for the watch movement.
The jeweler would then put the movement and the case together.
Opening wrist watch cases, especially ones made after around 1950, some times requires special tools.
Earlier wrist watches are often similar to pocket watches.