Stranger things have happened. The sprint package seem to denote a particular car. Sorta like a GT/CS. But some of the sprint options could have made it on a 6 pop GT/CS. May have been done at a dealer or who knows. People get to check that option sheet off any way they want. And the problems Ford was having moving the Mustang could make anything happen in that era. May have been ordered that way by a dealer to spruce it up or for a special customer in mind. Marti report could help, but it is not the end all of what happened to a car. May help and may add to the mystery.
Rob