The difference early vs late is the "return" that extends behind the early bumper vs the 90-degree flange for the late fillers. I would think that the drop-offs differ as well, the early being longer for the return to weld to. Seems that if you want the return (no gap), a pair of lower patch panels would provide the metal to be added.
I've found that parts catalogs are not absolutely accurate as far as application & usage. Sometimes WAY off !