Agree that the visible, from the trunk, ends were flat and had two opposing side that were basically straight or flat. There was a running change between an earlier and later version in 65 but these are not present on anything related here except for the carry overs which would have had the later version that carried over to regular 66 production. For the Mustang guys we can't pen down when the changed was exactly made since we don't know when unrestored examples were truly built but Ford states in the first part of March 65
67's also had a running change during their production (big surprise
and the visible ends on those were the round nipple style were are so familiar with and were often installed on the early cars during refreshing or restorations over the years
Not sure if they ever serviced the flat end versions for very long
A few shots of that visible, from the trunk side, view. Didn't note but the sampling is from both 66 and 65 and likely represent both the early 65 version and the later. The version lacking the markings is what I've found on 66's typically