Congratulations Bill. You have entered "The (Bolt) Twilight Zone". This is an example of the problems encountered when trying to document the different head markings on bolts. That particular bolt, 371041-S2, was used on 67 to 70 Mustangs (and possibly 71-73) exclusively for the power steering pump brackets in quantities of 1, 2 or 3, depending on engine, A/C or other variations. There could be assembly plant considerations as well.
I would say that if 2 or 3 were used, the head markings should match.
Jim