The diagram I have of the grill just has 42127-S.
What is the diagram you are using?
If you go to the Forum Library, you will find a topic called - "Mustang Hardware Spreadsheets". It lists many applications for 42127-S2, -S8 and -S36 which were found listed in all years of the Mustang Assembly Manuals. The "author" of this spreadsheet (yours truly) encourages you to take a look.
Both have CF on the head
I've found several head markings for 42127-S(x) and in general the zinc plated ones used on the grill are head marked "CF", with an occasional " "" " (I call that one the double ditto). Other 64-66 head markings are: "D" - as a -S8, used for grounds; "RBW" - as a -S2, used as grounds and as sheet metal retaining. I have other head markings used for other years applications: "circles", a "dimple", plain, and some marked "FC". There are some that have a flat blade screwdriver slot I've found used on 65-66 Fastbacks (no head marking there).
Jim