I do not think of it as much of a mystery at all.
End cap moldings on coupes/convertibles are indeed OPTIONAL, available only with the Exterior decor groupe IN THE 67 MODEL YEAR. In 68, they were standard.
Remote mirror standard in 67, optional in 68.
Rocker moldings are standard on all flashbacks (except Shelby) and optional on coupes and convertibles.
In the 67 model year, I do not think there was any date change-overs on these items. 68 model year there were several trim changes after the strike. A list of those changes are in another thread.
There is likely a high probability that owners may add on these optional moldings to make them nicer, not understanding they were originally part of an option package.