Here's even more detailed info (from the Decoding FAQ):
LOS - Fisher Body body scheduling code
The LOS (Los Angeles/Van Nuys) body scheduling code is a letter followed by one to three digits, e.g., F103. Based on data analysis only (there is no GM documentation yet available that describes this code) - but with consistent application to thousands of datapoints - CRG believes the code was used by LOS Fisher Body (and other plants that built multiple carlines) to aid in scheduling the body build order.
The letter indicates the approximate production day of the month for the start of the assembly of each vehicle body. It began with letter "A" on the first day of the month of the Fisher production calendar, incremented at the start of each additional production day to the next alphabetic letter, and was reset to letter "A" at the start of the next production month. (Note that the Fisher production calendar is known to differ slightly from the calendar month, and we also do not yet know exactly how the Fisher production calendar related to the Chevrolet monthly production reporting calendar.)
The one-to-three digit sequence number that follows the letter was reset to 1 at the start of each production day and was incremented serially with each body built by the factory as the day progressed. Data analysis indicates that the sequence number at the LOS plant was assigned regardless of model or body type. By this we mean that Camaros and full-size passenger cars both incremented the same sequence counter. The unit counter was reset to 1 the next day as the day-of-the-month code letter incremented to the next letter. In the example, F103 would indicate the sixth day of production for a given month and the 103rd body started on that sixth day of production.
