Bill, anything to add to this one?
I have a basic understanding but I think my understanding is tainted by poor samples WITHOUT confirmed Marti Reports or build sheets...I wonder if it was a "by itself" "stand alone" or if it required other "basics" in place to be added in a vehicle order form.
Sorry, I appear to be at the mercy of conflicting documentation.
I keep two versions of the 67 Illustrated Facts and Specifications Manual.
While waiting for my DP to harden I wrote this post using the shop version of the booklet, the White Cover version offered by Jim Osborn "MP294".
It lists the "Custom Exterior Trim Group" with the last sentence as "... on Hardtop and Convertible, plus lower back panel grille on 2+2 fastback."
The Blue cover version of the same booklet I have in the study, but by Mustang Monthly Magazine, strips off the portion of the sentence after the comma.
I've never seen many GTs with the rear grill and doubted the veracity of the White Cover version. Sloppy reporting on my part.
I'll update my first post.
By the way, $19.48 for the back panel grill in 1967 is $145.97 in today's money.