In my book "Mustang the original muscle car" by Randy Leffingwell its states that
"Its appearance reflects some of the subtley Carrol Shelby advocated in his Mustang's early days. The GT grille bars were deleted as was the pony, and the small, round fog lights were replaced with Marchal (later with Lucas) driving lights. the GT/CS package was a $194.31 option, which did not include the $146.71 GT package. In fact, it appears that fewer than 8 percent of GT/CS cars were also GTs."
So there ya go =/, funny thing is I bought this book yesterday