This is the one referenced by Ruppstang[/size][/font]
The screw (conventional wisdom calls it a screw) is shown in the "1968 Mustang Body Assembly Manual" (AM0021) pg 13 (N8-8105-4) as Ford Hardware part number 40923-S (no finish code shown). For 1969 it is also shown as -S (no finish code) but for 1970 it is shown as being a "-S2" (phosphate and oil).
I think that it is safe to assume that assembly line workers would or could use any fastener that fit the application based on inventory and availability.
Workers did not have that prerogative. Everything was done per engineering and assembly line documentation. Variations, like parts substitutions, needed to have approval of the assembly engineer. There are exceptions of course. Unfortunately these substitutions were not always formally documented.
Here's what you are faced with: the 1968 Body manual showing 40923-S, or pictures that have no referenced part number. Your call.
Jim
Jim