There was no official 1964 Ford Mustang. The first model released was 1965. Just as new model years today, these cars were released the prior calendar year, in this case quite early, on April 17, 1964. The 45th Anniversary model should have been the 2010, and the 50th anniversary should be the 2015.
The fact Ford instead used 2009, and probably 2014 respectively shows their acceptance of the “1964 1/2 model”. There were slight differences between the first run of 1965 Mustangs and the later runs of the 1965 Mustangs. The first run is commonly known as the 1964 1/2 model.