66 didn't use any covers on the A pillar, just a pinchweld molding on the edge adjacent to the door/vent window.
Are you talking about the headliner trim at the top of the a pillar? It's the little corner trim with the screw in it where the vent window meets up with the windshield in this pic.......... That's the only trim the a pillar used on a '66 or '67.
How far down the A pillar does that piece go I have a small piece in my car, I thought it was broken off. That piece is to finish off the headliner?