You are correct regarding the slots. The black plastic retainers slide into the header panel in the opposite direction for the 72-4 header panels than they do for the 1971 header. You absolutely cannot use a 72-4 header panel for a 71 car, you will never be able to mount the grille because of the placement of the slots, they are shifted over from where they need to be to attach a 71 grille to it. The 72-4 panels have a bracket spot welded to the "arm" of the header panel. That bracket holds the dual horns for 72-4 cars. The horns on a 71 are mounted behind the washer bottle. That is the easiest way to tell if you have a 72-4 panel or a 71 panel, look for the bracket for the horns on the arm of the panel.