I got my car in '72. It got stripped down for paint in '73.
What I found on my car was that all of the fiberglass had been painted with straight enamel paint, directly to the fiberglass with no primer used at all.
The car was low mileage at the time and had originally been a Hertz NY, NY car.
It had the "decals" on the hood in the indents.
There had been some touching up on the hood in the "nostrils" and on the lower nose with enamel but that is all that showed on the car. There had been no repairs showing other then that and there was no reason to suspect there ever had been and that the paint was original.
Whether or not that is consistent with other '68s, I can not verify. I have only stripped the paint off of one. Mine. -01107.
Opinions being what they are, just opinions, right or wrong, I would expect all original '68s to have been painted this way without primer on the fiberglass. The original fiberglass looks pink bare with just a hard epoxy like finish to it.
I do not know how service parts were delivered but it should be noted that Ford Service Parts often vary considerably cosmetically from production line originals.
You should consider that IO Smith was THE innovator in automotive fiberglass production at the time and that this was likely one of their innovations, i.e., no need for a primer?
How important is all this to you in that it is even unlikely that anyone is going to attempt to use the original paint procedure on "restoring" the car now?
Lots of cars have "interesting histories". Many having been heavily raced with little documentation and many have been back from the dead. Some are bigger challenges then others but that's part of their stories and "color".