I'm sorry I neglected to the X derivatives - X1 X2 X3 X4 X5 X6 X7.
X = black interior trim, which was used with blk std, blk dlx, blk htooth, white htooth, orange Htooth, white std, yellow Htooth, and any other color which had black interior trim.
RS/Z28 04C 711 blk std X3 clearly visible.
I have never seen a car with a color other than black trim have an X on the bulkhead.
If someone has a car with blue red or green interior which shows the X on the bulkhead vs a color abbreviation (R B ltG or G) please provide photos.
Lacking such evidence this issue doesn't have much controversy left.