I'm not so sure about the "CP" marked heads of the screws. I believe they would be "CF" markings. These are the same style and size as the ones used on the 64-66 grill, 42127-S8 (clear zinc), except they should be a -S2 finish (phosphate). These are the 14 used for to attach the vent assembly to the body. I've found that type on several of my Fastbacks in that application but with one difference, they were slotted as well as hex drive screws, no head marking.
To the original poster, I recommend you get a set of the Osborne Productions, Mustang Assembly Manuals for your year car. There is more than just 14 screws to attach the vent, there's calk, tape and plastic sheets involved, plus black out paint.
Jim
From "soggy" Northern California -