LV's Harrell Chevelle has the plain SS emblem and I think it's a safe bet to say it hasn't been touched, and the rust bucket '68 I posted pics of had the plain SS as well, and I can say
for sure it's hasn't be touched. Both cars shared the rare UAW strike interior so I'd bet that the build dates of these two cars are within a week or two of one another. LV...what's the build date on your Chevelle? Let's look at known unrestored cars both before and after these dates, then maybe we'll come up with a time frame for the switch. I agree you'll probably find all later cars having SS only, as it makes more sense to switch to a generic "SS" for all Super Sport models than to have two separate parts for such a trivial application. Wonder if late production SS427 Impalas got the SS427 horn shroud emblem, or if that was done away with too and replaced with SS only?