The 23 with star washers (believe that is what your describing) was used on some early (65-66) Mustangs for the shock tower supports.
I believe the head style requested is a toothed (serrated) disc washer, vs. external toothed.
The ones in the picture used on the shock tower to firewall braces are 3/8-16X1.0, 377917-S2.
In my collection, I find no 5/16-18 bolts with that head marking. The majority of markings I find for that length and style are the "Sems" in a circle, and the double "ditto" (or double quote). All this "may" indicate is that bolts with that head marking were not used on Mustangs, at least made in San Jose.
As Jeff stated, start a junk yard search. To give you an expanded vehicle search, if you can determine the plant, and then determine any other Ford or Mercury cars produced at that plant, you may find more usable hardware.