The trunk lids, as in 67, were mounted on the car. They were not painted elsewhere.
The clips were mounted to trunk and the P&O bolts held the lid to the hinges.
I suspect after the trunk interior and trunk lid interior were shot that the lid was held mostly closed with a device (wire?) of some type and the exterior shot.
Look in the un-restored section and you will find cars (incl fastbacks) that have paint on the hinges, bolts and trunk lid underside.
The only trunk lids painted separately off the car are the Shelby trunk lids (in 67). Others can speak to the 68 Shelby lids.