Here is a great reply I received from
www.rustreplace.com
Thank you for contacting us. The 1971/72 Grand Prix and the 1970-72 Cutlass SUPREME share the same rear window geometry. The deck filler panel may be the same between the 2, but we do not know this for certain. The Supreme deck filler panel is available in reproduction from both Golden Star and Sherman (likely the same parts made for Sherman by Golden Star). Good luck with your restoration
I'm a few steps closer to getting my rear deck filler panel.
6ft Hood