My main area of interest is 69 StL plant cars that carry something similar.
I don’t know that I have heard a definitive answer on the stamping. They tend to be pre paint indicating they were stamped early in the assembly process. This decreases the likelihood they were an inspection stamp and, in my mind, were possibly related to the Body In White assembly.
69 and later Lynch Road tags carry the GATE and BASE numbers for the jigs used to assemble the BIW. One of my theories/questions is: are the two digit stamps found on tags, as posted above, that particular plant’s equivalent to the GATE number found on LR tags?
The stamping from some plants are easier to read than others. As with anything else tag related, once you gather enough data from enough tags, patterns on how factories actually did things will emerge.