Back in 1964, I remember billboard ads for the 1964-1/2 Ford (Mustang). I really cannot see the differences in the two models. Via the magic of copy/paste, I've included below, the data I found on the subject...

In 1961, Ford Division Vice President and General Manager Lee Iacocca had a vision: a sporty car that would seat four people, be no more than 180 in. long, weigh less than 2500 lb, and sell for under $2500. On March 9, 1964, vision became reality as the first Mustang rolled off the assembly line. By the end of the day on April 17--the Mustang's official on-sale date--Ford had sold over 22,000 units, and the ponycar revolution was on. It remains one of America's most popular nameplates and makes a superb "first classic" and weekend playtoy.

1964-1/2 Mustang

1965 Mustang

Ralph's scan