The Shelby GT 350 S, first non white GT 350, it was the Paxton prototype. By the time production would have started the idea changed to an option

found on http://www.paxtonauto.com/article.php?id=3

Click on the image for the full size to read the writing, but basically it's a publicity-press release shot in the event that the GT 350 S were put into production, like the GT 350 H (Hertz rent a racer program)