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.
Here's a link to a PDF supplied by Alex. It lays out in detail the points he mentions above.
This will help others follow and participate in the discussion, with graphical examples and tables which would not otherwise display properly on the forum boards.