I drank beer one night with a guy who worked for GM in the late 60's through mid-70s. We talked about fasteners and some other parts. He told me, that with a few exceptions, most of the fasteners, etc. were bought by the millions and that a renewal contract with a different supplier might mean a change in the fastener and how it looked. I would think that with that to overcome, I would be happy with anything that A: fit correctly & B: looked close to the original.While, the purist side of me agrees with your passion on this, you know that isn't 100% true. You CAN find some of the hardware in odd places. Granted, not going to get that special length bolt or certain head stampings, but in the case of things like locking crimp nuts, and some washers, etc., you can get them from general sourcing. Especially if you stumble across a supplier who supplied GM with said fasteners. But all this only works if you already know what the fastener looked like from the factory. The way I see it though, is if GMSPO has a different part number to a particular application, and you use that GM fastener, even if it didn't look precisely like the fastener that was being replaced, it is the "correct" part. Fasteners seemed to change part numbers like I change my underwear, about once a month.
I will attest to the bolts being the majority of the problem when finding the correct looking parts for a restoration. You do not always have to go with used OEM hardware. A correct-sized phosphated washer head body bolt holding the bottom of the front fender to the car is likely not going to be noticed, even in a 1000 point car show. Obviously if you can easily see the bolt, like in a valve cover or intake manifold, and it doesn't look correct, you will likely get dinged, but for the most part, a fastener that blends in is usually not flagged on a point sheet. Most judges probably wouldn't even know every manufacturer's marking for every year anyway.