I noticed the belts but may have missed your point. I have seen red belts in red interiors too and in '68s, including the '68 brochure.
I don't question that they should be red. I question why mine got black instead. Maybe those two QC inspectors in the brochure did not inspect my '68.
Maybe they liked the contrast also?
(ok, ok, the two QC inspectors are one blonde and one brunette)
1968 400 Coupe, verdoro green, black vinyl top 1968 400 Convertible, verdoro green, black top 1971 Trans Am, cameo white, auto 1970 Buick Skylark Custom Convertible 350-4(driver)
