Core support "Blackout"
I know this is an old post but the information is still valid.
This is my original valance (1967) that had the inserts painted black, I know my Grandfather (original owner) didn't paint them.
On my car when the blackout was done there was over-spray onto the fender on the right side.
On mine the latch panel had no blackout on it and the rest was hit and miss.
You can also see how far the factory undercoating came up the floor pan (I removed it back to the spot weld joint)
The blackout behind the grill is something that probably varied widely between cars.
Valance should have the inserts painted (at least in '67) I can see how some cars may have been missed.
Alan