All three of the parts you mentioned are the same part for 67 & 68 Mustang, but do not cross over to other Ford vehicle lines which will reduces the potential field.
I was hoping to see one or more parts also used on something else. I don't have the Mercury MPC but would think that these would cross over to the Cougar.
Looking back to the future, to see if any of these were used after 1968, I came up empty handed.
Good luck with your search.