It's not a cover, it was an australian package called the Ford Capri Clubsprint. 400 were made, they had a specially designed body.
SSV (sydney special vehicles) makes a reproduction kit that's fairly close to the original, but it's expensive and shipping is absurd. (look under Ford as they do not have Mercury brand over there)
http://ssv.com.au/ Russ made a prototype "half-sprint" kit, which instead of replacing the entire rear bumper (clubsprint is like this) it was a panel that just replaced the tailight portion, & used the 4" round corvette style lights. He has not put this into production yet, unfortunately
Btw, welcome to TeamCapri! Always good to see new folks from PA here too