I got curious enough to go look, and agree with Midlife.
However, the following years still had the side "C-scoop" indentations, just with different forms of trim pieces in the area. The '64 1/2 - '65s were usually unadorned, the '66s had a bright metal piece with a couple of scallops, the '67s had a 'fake functional intake vent,' and the '68s went back to a simple bright piece with scallops. The "C-scoop" indentations went away for '69, but still had some newer 'fake functional intake vents' in the same area on the hardtops. They finally went away altogether in 1970.
The devil's in the details. In '67, they changed the Tri-bar Pony and Mustang letters on the fenders - stretched out the letters and lowered them. Then changed to the cursive like 'Mustang' script letters.
Interesting how they changed this or that from year to year. I know [in general] we can usually pick out the subtle differences between our '71-'73s, but maybe not notice as much details with the other models, and then seem disappointed when the VMFs can't tell the obvious things apart... then see how they get irritated when other can't tell their favorite year models apart (shoe being on the other foot, and all). I guess that's the downside of having so many split-off factions from the mainstream.