Those are buck-rivets. With a bucking bar, which has a small divoted end, to match the head of the rivet, and an air hammer, as well as, another pair of hands, you can set those, to look like the originals.

Tony Smith
4-5-6 Chevy Trucks
501 Scott Avenue
Kansas City, KS 66105
913-207-7789
[email protected]
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Custom Parts: http://www.flickr.com/photos/4-5-6chevytrucks/sets/72157627022818563/
www.4-5-6chevytrucks.com