Factory curtain is a separate section of top that has glass window welded in place, as well as aftermarket plastic window stile curtains, and is fastened to metal at trunk along gutter rail by screws, and held by zipper to a length of fabric which is stapled to the rear top bow.
Most top sellers offer plastic window curtains for these cars as a lower cost alternative to OEM glass. Both glass and plastic stile curtains can be purchased separately from full replacement top package, and install the same way.
Chances are good if the rear curtain is badly weathered with rotted zipper & window, top padding and top is also in poor condition as well. At a minimum replace top padding in conjunction with curtain and get a few more years of use out of the old top, or replace the whole shoot’en match; top, cables, tack strips etc.