1968 Dart GTS grill center

there is still a lot of mystery as to what cars had the grille center ....my cousin has two factory GTS 68 darts here .....mostly original and one had the center and the other one did not .....the earlier of the two cars has it and is a big block car.... ....the one that does not have it is a low option small block car......any correlation ????

i have wondered what was with that for a long time now