In 67 the dual cutout rear valence with quad chrome tips was, per my understanding, only included from the factory on GT/GTA models with the 289/271HP and the 390.
GT/GTA models with the 289/200HP and 289/225HP received single exhaust with a turn down before the rear valence. In other words, not all GT/GTAs have the cut outs and quad tips. Hard for some people to believe.
The non-GT based 390 received dual exhaust with turn downs before the standard rear valence - no cut outs, no chrome quad tips.
The non-GT 289/271HP is a trick question because the GT(GTA) option was required to get this engine, thus the first sentence applies.
As for what a dealer would do that should be left up to the imagination, unless you are fortunate enough to have paperwork.