I have that same "LG7" yellow crayon marks on my rear bulkhead. "LG" is "Light Green" interior color. "7" is for the x-77 trim. The car I have was a light green color custom trim level car built 10A of '68
"My" interpretation of this mark - The Fisher body line needed a clue of what color to spray the interior areas that are visible once the car is finished. The light green interior color will show through the air vents and probably other places so they had to paint that before the interior trim was installed. THe CRG has not weighed in on this but that is my best guess. You and I are close in build week so it makes sense to me that they were marking them that way at that time.
The black magic marker lettering is a new one to me. I doubt it was factory markings but who knows.