'68-'69 Barracuda Ralley Dash only

I know it's not 68/9 cuda, but I have one from a rally dash duster/demon and it has that plate. I've seen them on all years of rally dashes. Like u mentioned, weird how it's not in parts manual.
Just peculiar that only a Rally dash requires it. They only have one extra gauge and that would be the vacuum gauge or tachometer. Am I correct in that thinking?