These plates are extremely hard to find, and were also used on the '68 Shelbys (not '67). Most likely there were made at A.O. Smith, along with some other metal parts, by a side company called "Mitchell-Bentley" that made smaller trim parts, such as your cigar lighter bracket.
I haven't seen the ones in fiberglass. Interesting.
Since these are so difficult to find, one could look for these in the trunk as part of the search for authenticity. Clones usually don't have these, and that is why the owners don't like to open the trunk to show anyone.
"IF" anyone wishes to reproduce these, note that at the base of the sphere, there are buckling-type marks from the pressure of the stamp vs. the thickness of the metal. The stamping has also distorted the square shape of the large piece, too.
These were painted a semi-gloss black, just like a lot of other black parts originally on Mustangs, Fords, of that era. Too often, repainted CS cars have body color covering these plates when the painter just sprayed into the trunk.
Paul N.