F= Mustang (Ford coding for all Mustangs) on the line so IMHO its likely an identification by the producer that the radiator support is for Mustangs
With that being said don't recall seeing one (the F) on other 69's - was just sanding in that area a few days ago - spring 69 Dearborn panel. Has the radiator support been replaced? Don't see the spot welds in that area of the F. If not what is the date code (if you please) from the panel?
As a side not you'll find at some plants (ones that also produced Cougars on the same line) that they marked the front of the cars (before the sheet metal was installed) with a "F" in crayon so that workers knew the body was a Mustang not a Cougar since they both look very similar from the front