It was an option to have the trunk lid trim, but I think it might have been standard on fastback, like the rocker panel moldings were. Coupes and verts came without it unless otherwise ordered. (Someone feel free to check me on this...)
It came if you ordered the Exterior Decor package also.
Correct jfstang, it was part of the exterior decor group which included the louvered turn signal hood, bright rear deck trim (on coupes & convertibles), pop open gas cap and bright wheel opening moldings. I think it may have also included the rubber trim on the front bumperettes. I also believe the bright rocker moldings were a separate option.