I have, like Vince (302 Z28) purchased hundreds of dollars worth of stainless fasteners from McMaster-Carr and have yet to see any rust on any of them. More have come from Stainless Specialties and a guy I see at swap meets here in the NY/MA/CT/VT area. Never a bit of rust on the 300 series SS tho some that are 17-4, I believe (from elsewhere) do show some discoloration, but no rust. My guess is that what are on that car are what came with the molding (especially if it's replacement aftermarket) and are probably chromed carbon steel.
Here's a good chart to start with, drill down if you need more: