it appears that the 68 used a 'C' branded fastener for the short bolt (374706-S(2)) going into the frame inserts and the 'F' branded long bolt (374420-S(2)) for going through the frame.
In my stash, all 16 of the 374420-S(2) bolts had the "F" head marking. However, there were three different grade markings and four different styles. There were the standard grade 5 - three marks at 120 deg; some grade 7 - five marks, some at 72 deg, some "squished" upward, still 5; and grade 8, six marks at 60 deg. All these bolts came from salvaged cars so I can't tell the years' application. I assume the grade 5 bolts were early years, 64-66, but after that I don't know.
But, 374706-S(2) bolts had no head marking. There were only 2 in the box.
Jim