Just me but I don't recall ever seeing/finding the large flat head screw used to attach the splash/light shield to the car. Have maybe a dozen original sheilds and none have an imprint from the larger screw (see pic below). BTW AMK is great in a pinch but the style and head markings they can fin dtoday often do not match what was originally used at all the plants - just my experience
Here are two pictures from cars from Dec 66 showing what was install during that period (both are consistent with the other). Appears all used the same screws during this period fro all of the valance screws and the plash/light shield
Third one in from the end painted
Jim for 65-66 the type of screws used depended on the time period - not all like the "book " might indicate. Also the pan heads have been found on original cars in phosphate also on original cars. Just what I've got pictures and notes of
There is one thing that is constant, and that is Ford's inconsistancy.
I helped a friend restore a 67 A code Fastback built May 67 in San Jose. As in any disassembly and reassembly there are surprises. One surprise was the rear valence that was attached in the way I described above, two wafer head screws and nine pan head, the same way as my two 66 San Jose cars were done. Ford Design Engineering specified 11 screws, Ford Assembly Line Engineering specified two "odd" screws to hang the panel for painting, and then the original type. I can't remember if the 2nd or 3rd screws was the wafer head ones though. The big difference was the size of the nine screws, 8-18X1/2 for 67 and 10-12X3/4 found on my 66 cars. Why do I remember this? The 66 Body Assembly manual calls out the 8-18 screws so I ordered them from AMK, only AMK didn't carry the drill tip style so I ordered the standard pointed type. When I got around to using them, they didn't fit, too small, so I resurected the old one for a look-see and found they were the 65 type, 10-12X3/4 round/washer head. The 8-18 screws went on the shelf, but got pulled out for use on the 67.
Jim
(Note: in the 64 and 65 Body assembly manuals, the assembly line instructions are to match-drill for installation, the 66 and 67 Body manuals don't have that note. As the 67 type was a drill tip, the pre-drill step was eliminated. It may have been intended for 66 Mustangs as well, but use up the old inventory first.)
(Another note: pan head screws as used by Ford or supplied by AMK don't have head markings. It's the threads per inch that are not always available.)
(One more note: from my "notes", a different type of screw, 10-16X3/4 Round/Washer (not listed in the AMK Guide) was found on a March 66 San Jose GT Fastback. The car now speaks Australian.)