It was a long time ago so perhaps you’ve forgotten that Mustangs never had a 4 cylinder (170 six, 260 and 289 eights) and the first Mustangs went on sale in early 1964 with post July 1964 units labeled as 1965’s. (hence the ‘64 1/2 monikers ). Perhaps the easiest way to identify a “64 1/2” is that they all used a generator instead of the alternators used in post July 64.
64 1/2’s didn’t have backup lights either. Did you know the first 500 production model Mustangs had bench seats?
I think that a few of these Mustangs had four bangers.
You know more about the car than I do. All I remember is that I bought it in 1964 for the outrageously high price of $2,250 just in time to impress the girls at the summer semester in college. I don’t even remember if I was successful doing so.