This was always my assumption. Take the 72-74 Challenger tail light assemblies for example. I doubt the factory paint shop had time to mask the recessed tail light openings before they sprayed the argent. My guess is the tail light housings arrived at the factory as complete assemblies.(already painted and lenses installed) All the factory had to do was spray the argent on the tail panel.

Look at modern car bumper covers. Most do not exactly match the body color. They are painted by the manufacture and shipped to the factory as an assembly.