Beg to differ slightly on your explanation of the code shown ahead of the SO number. This is actually known as the Body Frame Date. This is the date on which the unibody shell was built. It can be, but also may not be the date on which the car was driven off the end of the assembly line. All assembly plants in this time period maintained a body bank of painted bare body shells within the plant. Shells were kept there for a variety of reasons.
Colour: The plant tried not to paint successive cars going through the paint shop in different colours. The paint guns needed to be blown out every time a new colour was used, and by running two or more cars of the same colour together it saved them paint.
Options: If a particular car were ordered with a large number of options, the shell may have been held until production control could confirm that everything necessary to build this particular car was in the plant. A lot of white shirt type people were not happy when a number of cars were built and sent directly to the major repair bay to retrofit something that was not available when the car was built.
Complexity: If a particular car required a higher labour content than was considered normal, it made life very difficult for the people on the assembly line to build a number of these cars consecutively. Station wagons and convertibles immediately come to mind as falling within this definition.