1964½ is a term that is used by Mustang owners and enthusiasts to describe the earliest Mustangs ever built.  Although Ford never officially used the term "64 and a half" to describe these cars, they did arrive on 
the market in early 1964, before other true 1965 models. The first Mustang actually rolled off the Dearborn, Michigan assembly line on March 9, 1964. This was in the middle of the normal 1964 model year, giving rise to the common term "1964½" Mustang.

These early cars also have a large number of important differences from the true 1965 model Mustangs.

Basically, if the car was built before August 17, 1964 - it will have come with a generator instead of an alternator, and therefore be considered a 64½ Mustang.

Officially, Ford stamped a 1965 VIN number on all 1964½ and 1965 Mustangs.

1st Dearborn 64.5 Mustang: 5F08F100001 
1st San Jose 64.5 Mustang: 5RXXX100001

1st Dearborn 65 Mustang: 5FXXX250001 (1st known fastback - 5F09K250009) 
1st San Jose 65 Mustang: 5RXXX125001

Many changes came before the cut off date for 64.5 production (Sometime in early August, 1964.) and many came well into the regular 65 production.

Ford did have a shutdown period for retooling of the Mustang lines in early August, 1964. This is when the switch from generators to alternators was made. When they started back up, the VIN numbers were rounded up to 5FXXX250001 at Dearborn, and 5RXXX125001 at San Jose. To those of us in the hobby, this is the point when the 64.5's ended and the 65's started. There was a lot of overlap before and after the shutdown.

Not much and yet everything. From 50 feet away, they look the same; close up, they host quite a number of differences. The ’64-1/2 Mustangs have always been ’65 Mustangs--always. As enthusiasts, we call them "’64-1/2" because it’s easier to say, but it’s also a cult thing. Early ’65 Mustangs have the distinction of being the first block of Ponies ever produced. They were an integral part of the Mustang madness that swept the world in 1964. And if you have a "’64-1/2" built at Dearborn, that makes it even more significant in the relative scheme of things, because it was born in the motherland.

When the Mustang entered production early in March 1964, it didn’t have many of the refinements it would have later on; call it a reskinned Falcon with bucket seats. To be a ’64-1/2 at all, a Mustang has to be factory-equipped with a generator charging system, a 170ci six (U-code), a 260 2V (F-code), or a 289 4V (D-code) low-compression, large horns mounted down on the frame behind the radiator, a brake light pressure switch on the master cylinder, a center "off" heater fan switch, and a generator charge light, just to name the basics. These are features exclusive to Mustangs built between early March and July 31, 1964 (scheduled build date code only). The ’64-1/2 Mustang is simply an early-production unit without the refinements that came later. And another thing, Ford never built a production ’64-1/2 Mustang fastback...evar.