I can take a picture of the 'F' bolt from the original 67 LOS car to show the weathered age and that should help prove that period usage. The restored NOR car was completed in 1986 (took 4 years) and I don't think there were any reproducer of bolts at that time
and the 'F' bolts pictured here were restored using black oxide chemicals back then. They stayed well preserved because I use to apply oil twice a year to each bolt head for many years after I stopped showing the car. These aren't today's repos and I'm sure the kits today copied these original patterns.
The survivor 67 LOS car is completely original , including bolts, with the exception of REAL dated 1974 plug wires (and plugs likely) on the car when I bought it in 1980.
This whole topic sounds like a good research project. I'm sure a data base can be compiled and bolt marking patters quickly established and even narrow down period correct markings if the sample size is large enough. Tonight I thought of documenting each bolt marking to the manuals part number as an ad-hoc winter project.