Barn find 72 340 duster

Tim! The second picture in reply #117 there is a boot/plug. They were used by the assembly line workers to get out all the air of the water channels after the cooling system were filled.

On some engines these plugs were mounted before the engine was painted a.d some engines got unpainted plugs after the procedure was done. I guess they were not mounted in time or that there was a change in production. It was faster to not mount the plug for the guys who took the air out of the engine. Otherwise they had to take away that plug when bleeding the system.

I think you are right re the one piece mounted as gasket. A great source is to check out old magazines with color photos of the engine bay. It's a great resource for information.