Technically speaking, there is no such thing as 64 1/2 Mustang. Ford unveiled the Mustang in the spring of 1964, half a year in advance of Detroit's traditional fall start to the new model year. But they were advertised and sold as 1965 Mustangs.
The 64 1/2 designation is a modern hobbyist's concept, generally applied to cars built prior to mid-August 1964. August 17 is often cited as the definitive production date. Cars built before that date are considered to be 1964 1/2 Mustangs, cars built after are 1965s. You can identify a car's production date by decoding the data tag on the inside edge of the driver's door.
There are no 64 1/2 fastbacks, only coupes and convertibles. The fastback was not introduced until September.
Other tell-tale identifiers include:
Ford gradually introduced changes to the design throughout the year, but not uniformly. Existing stock of parts would be used up on the production lines before replacements would be used, and different manufacturing plants implemented different assembly practices and schedules. This means that the absence or presence of any given marker is only a guideline, not definitive proof.
For additional information, check out the Vintage Mustang Forum's page Mustang FAQ and How To's.
Also, see this discussions on the VMF:
submitted by a65ragtop