I agree with Pete. Here is some additional info and pictures.
We are talking about what the Ford buyer's digest calls the "lower back panel grille" which cost a whopping $19.48. The option for "two tone paint - lower back grille" was $12.95. I wonder what the production statistics are for these options.
There are two styles, the coupe/vert version and the fastback version. The coupe/vert version has a drop along the top to accomodate the trunk lid style. The fastback version is straight across the top.
The coupe/vert part numbers are C7ZB-65403A76-C and C7ZB-65403A77-C.
Regarding colors, the two tone color used was "dark gray poly" - refer to this website -
http://www.svs.com/zim/mustang/images/1967-ford-ppg.jpg Also note that the two tone color was not limited to the grille itself but also the panel beneath. On this (fantastic) site do a search on "lower rear grille" and look at the picture posted by Jeff. The posting started on Aug 26, 2009.
The picture below of the blue mustang fastback has the incorrect color (style steel wheel charcoal gray) and does not have the back panel painted. I included it to show the difference between the coupe/vert and fastback styles.
As the pictures below indicate, the grille mounting was a combination of threaded rods at the tail lights, and studs in the center and near the gas cap. There are six mouting rods and four studs per grille. The studs are what Pete stated are often broken.
The stud attaching hardware includes a thick washer cut at an angle to account for the curved panel upon which it is mounted. The nuts are capped. Pictures of this are also shown below.