"I'd wager that the six-panel taillights were a "feature" that was reserved for the GL and Bertone-type models in the early days, but then adopted for even the lowly DL model later because it's cheaper to procure only one version across the (sedan) model."

I'd bet alongside of you. It started in '79 with the 6-panel distinction being awarded to those who bought six cylinder sedans. By 84 all trim levels got 6-panel tail lamp assemblies with the flexible circuits.
--
Art Benstein near Baltimore

“The secret of change is to focus all of your energy, not on fighting the old, but on building the new.” -Socrates