Johnnie,
The screws, 42127-S8, for the grill (6) and fog light brackets (6)are all clear zinc plated, and should all have the same head marking, San Jose cars have CF. The fog lamp brackets may have a different head marking as they were installed at a different time. The same screw is also used for the lower radiator grill bar (5) attaching. A nut clip is used on all those screws above. As you have one "left", check the grill bar to the hood latch support in the center.
I notice a rubber seal on the radiator support. It was only used on I-6 engines in 1966. Just a heads up as fog lamps were a rare option for six cylinder cars.
I also recommend that you get a set of 1966 Mustang Assembly Manuals (plus the 64-65 Chassis and 64-65 Weld/Sealant). All the questions you asked about the screws are covered there.
Jim