"Because the Mustang was introduced in 1964 but advertised as an early 1965 model, it is often referred to as the 1964 1/2 Mustang. Initial production of the second round of Mustangs began on August 17, 1964. Production included 92,705 standard coupes that retailed for $2,320 apiece and 28,883 standard convertibles that retailed for $2,557 each. Both the original production Mustangs and the second-run vehicles are technically considered 1965 Mustangs by Ford."
From the Ford Mustang historical record
"Because the Mustang was introduced in 1964 but advertised as an early 1965 model, it is often referred to as the 1964 1/2 Mustang. Initial production of the second round of Mustangs began on August 17, 1964. Production included 92,705 standard coupes that retailed for $2,320 apiece and 28,883 standard convertibles that retailed for $2,557 each. Both the original production Mustangs and the second-run vehicles are technically considered 1965 Mustangs by Ford."