The problem with GM fasteners is that the replacements, even if GM, are sometimes not the exact same in appearance as the factory-installed pieces.
The nylon washers for example. I'll bet dime to donuts that you won't find any GM parts reference to said nylon washers under those screw heads. Although every G-body Olds came with 3 of them, maybe 6 total with the nylon washers? I can't recall specifically. Been a good long while since I took one off. The front ones under the glass trim bolt on to the retainers (black plastic, p/n 20655818) which are bolted onto little pedestal brackets underneath and a "flat spot" on the retainers butts up against the base of the glass. Then there's the rear 3 that you can see by popping the trunk, and I believe they bolt to a little metal tab. Again, 3 of those. Total of 6 bolts to hold the panel on, and 3 that you can't see holding the retainers on underneath.