Let me add to the confusion. The bolt in question has a name. It is 379068-S (natural, no finish). To further the confusion, I have determined that the head of this bolt used on 1964 thru 1966 Mustangs were unmarked. No manufacturer or grade markings are on the head. My samples are mostly San Jose. In 1967 (early, mid, late?), a "F" head bolt was introduced, with a grade 5 marking. There are rumored to be other head markings. One I have is a "B" in a "U". That is identified as a reproduction. If so, it was sold at least 20 years ago. Remember, the majority of cars i have access to in the San Jose area were made in Milpitas.
Jim