1970 R/T Grill Question

Hi all I have a set of grills that came on my car. One original and one used in good condition that I will be using.

The original one has a red line around it, inside the lip on the front of it. I have seen these a lot on a few differnt Coronets not just R/Ts but Bee's as well.

I wanted to know if the R/Ts came standard with that or just what ever was laying around?

I like the look and think it would bring out the black body of my R/T.
Author: 70CoronetR/T