Yes, the line workers do create the tags. The jobsetter made them and sometimes the foreman didn't even know about the change in a procedure right away. The highest paid hourly worker is king of a line. The lines were/are always evolving. If someone has a good idea it gets implemented. These days you get paid for a good idea like coding a tag to save time. Here is a known example of a line evolution in the same era: For example, for only half the year GM put certain codes on 69 Camaro trim tags. These were X codes. Very similar to the 3F code on a Firebird. By 1970, they quit doing it. To further complicate the X codes, only one plant did it. The other plants had their own codes that meant the same as the X code. But not all the cars at the other plants got the 'like X' codes. Their codes also were only used for a short time frame so there are many cars without codes. And to even further complicate it, one particular X code could mean more than one style trim.
On a Chevrolet new car lot in 1969 you could have two identical 69 Camaros that had different codes on the trimtags. More likely would be one of the two twin cars with a code and one with no lower codes at all. It is entirely possible that these two twin cars could have been manufactured at the same plant just days apart.