Hi Mate,
My take on this.....
My mustang has build date of May '64 and is considered an early model. What I've learned of the years, those musangs assembled from March of 1964 and lasting until late August of 1964 are considered "early" models. Where it gets a little crazy/hazy, is toward the end of that period, models could begin to have a mix of both early and late parts.However, they are all registered as 1965. We have added to the confusion for novice owners by referring to the "early" cars as 64 1/2.
Happy Motoring....