I believe all you say Steve. Argent caps were installed on your car at the dealer when it was new. The question is, was that normative for 68 Camaro at that point in time? Dunno. I believe most people believe - with photo evidence to support it - that for most of the year the black ring version was normative. I'm not going to rule out a late year change (as you are suggesting regarding your grille), but there are enough other possibilities that a lot more research would be required.
Not fully related, but as example of how confusing it can get, I just re-looked at "Chevy's Classy Chassis", a photo essay of nominally original S. Cal cars published in 1981. So at the time the photos were taken the cars were no more than 13 years old (probably several years younger) and most with the original owners. The principle 68 SS/396 they document has the non-SS P01 wheel covers on it. Definitely non-normative. An after delivery add-on, or a dealer mistake? Who knows. Car belonged to an Ed Boortz of El Cajon Cal, if anyone still knows his whereabouts (he's not in our db).