Some more details on the difference between 67 and 68 hinges would be useful.
If there are any known sources for the spring engineering numbers, it would be interesting to include. Service part numbers can be generic in application sometimes and not consider running changes.
Also, there were changes in the date format starting sometime in '66. I saw the different pictures, although noting transition times to the different date format could be helpful.
The finish of the rivets is something that comes up now and then. On unrestored cars, the rivets may be lighter color than the phosphate spring parts. It would make sense that the individual parts were phosphate coated and then assembled. I have no supporting evidence on that other than personal observations over the years.
One other thing, I have found small stamps, I don't recall exactly... like a 1 with a circle or similar.