Looking at the many movements Bulova used in the early days, al having a serial number, I wondered if there would be a system behind the numbers. After putting the most used ladies movements in a spreadsheet and analyzing them with pivot tables, I found out that there is a system.
By the end of 1930, the most used movements for ladies were the 5AP, 6AP and 6AF that were part of the main 5 and 6 families as I call them. In below link you will find a PDF that explains the system I found.
I used my findings to filter out typing mistakes in inputting serial numbers in my spread sheets. I hope this forum will start further research on serial numbers so the logic of other "work horses" like the 10AN can be discovered and added to this forum.