James,
For reference and clarity:
I have a 10A of '68 car. Empirical evidence seems to show many later production examples have "X3" or "X7" or some other X followed by the number character written there. No mention of the interior color code in the grease pencil marks later in the run. Not scientific, but that is what I have seen. I assume another process for communicating the color selection was in place later.
CRG references do not have any mention of this subject. Jerry's books have no mention on the subject either. I have been looking at this for much longer than I want to remember. Going back at least 2 generations of the CRG web site.