I've sold several sets of NOS "MUSTANG" over the years. NOS ones taken from the Metuchen plant by a retired line worker and the Ford Service replacement emblems. The assembly lines ones had the "engineering #'s" in back of those. The Ford Service ones from the 1970's and 1980's didn't have any markings cast into those. My guess? Ford had additional casting dies made and probably contracted an outside supplier to make the tooling. There's no way; one set of "C5Z" casting dies supplied emblems for every Mustang model year from 1965 to 1970 without taking a beating and forming castings with noticeable irregularities.