Black paint treatment under sill mouldings

This is for the 1970 Duster owners. Went to look at a new friends 1970 340 Duster, it had chrome rocker mouldings with Black paint underneath it. I asked the owner if that was factory installed and he said yes. he said to check the 1970 Hamtrack Registry and I found this under Custom Sport Trim Package for Dusters. It says Custom Sill Moulding with black paint treatment under mouldings. I never have seen this before, does anyone have this on their car from the factory?