You mean the rear seat area supports were not cut away for the fold down seat option on Firebirds?

My 70 Mustang Fastback did not have the folding seat option, but the upper rear seat cushion was mounted to a metal plate with hinges. After removing a section of trim at the edge of the package tray, the seat would, in essence, fold down providing complete access to the trunk (similar to my new Mustang) The back of the hinged plate was black painted metal and formed the barrier between the trunk and the seat. In other words, "most" of the folding rear seat option was installed in all the Mustangs

I assumed the Firebird folding seats also allowed access to the trunk.