64 1/2 Mustang is called that because as you all know it was released in April of 64...
Funny thing is the Baracuda came out 2 weeks prior to the Mustang and it is simply a 64. In 2005 the re-designed Mustang convertible was released in April..infact I believe it was even on the same day the 64 1/2 was released (just in 2005) and it is still called a 2005 (The Coupe was released in Dec of 04) Funny how Marketing Dept's play with this sort of thing...