One thing that you need to understand about AMK is they supply only what they can get made. Correct or matching head markings are one area that is lacking as many of the original Ford suppliers no longer are in business, so substitutes are listed. The same goes for the length of a bolt, many times shown with a disclaimer of "similar".
In my unfinished hardware collection, most of the cowl bolts have a head marking of "23". I also have some with a "26" and some with an anchor. These are the correct size, have the correct lock washer type and correct length (1.0 inch), so I assume they were used by one of the assembly lines at one time. My hardware data base lists only 64-66 application for that bolt, 377917-S2, no other Mustang application. I cannot determine year or plant as they came from a salvage yard that specializes in Mustangs. I have no head marked "SEMS" bolts from salvaged Mustangs, but that does not mean that they were not used.
Jim