My dad had a '73 Eldorado and it came with the hard parade cover or boot.
He never used it.
I found it when he sold his house, still stored in the strange carpet like bag that matched the trunk lining.
I'd say its new old stock, but i think it has some small amount of damage on one of the corners where it rubbed against the wall where it was leaning.
I will take some pictures if anyone is interested in buying it.

I will sell it cheap, but you have to figure out how to get it from me to you.
It is kind of heavy and large to ship.

I am in Pittsburgh, PA

Let me know if you are interested by sending me a PM here.