This is the original Nut and Bolt from my '65 6-Cylinder Dearborn Coupe, January 1965. The Reproduction Bolt Head is close but lacks the Ring at the outer edge and their Nut is nowhere near the same. I've seen your Bolt as well on other Mustangs and may be correct for 8-Cylinder cars or possibly different based on the Supplier, Running Changes, when Built, which Plant, etc. Hopefully someone else can add clarification; I'm no where near expert enough for this kind of detail.
Thanks for posting.
Ray