This is from an earlier post I made about this but the info came from an article in the Old Cars Weekly a few months ago. It was a neat article by a guy who had worked his way up at GM from a "grunt" to a management employee. I wish more of these guys would step forward before its to late.
For what its worth:
...and a recent article by an assembly line worker from the later fifties said sometimes it was the previous workers initial signifying his job was done.....in one case the markings were under the fender on the inner fender and to keep from bending over they would hold a mirror under the fender to see it.
Many have reported that some marking also indicate trim, engine and so on, or other options as it went down the line.....and that it may vary from plant to plant.
