OP asked if this was a paint code and no one addressed that question.
Excuse my prior post - Now I understand what you were referring to but guess none of us have really addressed the color of the mark as of yet, which I think is his need at this point. Just don't have many pictures and documentation of Cougar cars and not sure if it would have been the same bar as used on some Mustangs for a cross reference.
Looking at other buildsheets the "X" in this case may identify a specific sway bar rather than just the fact that the car was suppose to get one. Mustang sheets have a number of different codes that appear to relate to the engine size and/or suspension package if they got one at all. "C"s and "D"s are what I've found on those
