In a perfect world there would be old GM documents still around indicating fastener suppliers each year at each assembly plant. I think the first step is to ID the manufacturer head marking along with details of car it was on (model year, plant, Fisher build week, survivor or partial restore or fully restored, p/n, size and finish and notes like captured washers, how many required versus how many found on the car, etc). Not to say this is always the case, but I think there may be some newer fasteners/manufacturers that have been used on so many restorations that they are now assumed to be born-with correct. And even on a survivor a bolt could have been replaced along the years. Another obstacle is where do we find a definitive listing of fastener suppliers and all their head markings, especially from the late 1960s? Once that's done you could look at the identified manufacturer and see if they pass the logic test (existed in 1967-69, used that logo in that period, were they a GM supplier at that period, where their plants were located, etc). Would require contacting the manufacturers, and there will be gray areas such as suppliers no longer in business, suppliers that were acquired by another company since then, non responsive suppliers, or lack of old info, etc). This will weed out some manufacturers. Then add in the knowledge of 1960s GM suppliers from our experts who were there. You'll still have variances due to the "bin of mixed vendor bolts" they had on the assembly lines, but I think patterns will emerge as far as specific manufacturers used and their proximity to the assembly plants. For example, it seems like the ones we can already confirm were located in Ohio, Michigan, probably Illinois and maybe California. Unless it's a special fastener I doubt you will confirm use of a fastener made in Maine or Texas, probably not even Canada from that period, and I suspect we may be down to fewer manufacturer's than the number of logos we currently have.
