Since it was introduced five months before the normal start of the 1965 production year and manufactured alongside 1964 Ford Falcons and 1964 Mercury Comets, the earliest Mustangs are widely referred to as the 1964½ model. Nevertheless, all "1964½" cars were given 1965 U.S. federal VIN numbers at the time of production, and - with limited exception to the earliest of promotional materials - were marketed by Ford as 1965 models. The low-end model hardtop used a "U-code" 170 cu in (2.8 L) straight-6 engine borrowed from the Falcon, as well as a three-speed manual transmission and retailed for US$2,368. Standard equipment for the early 1965 Mustangs included black front seat belts, a glove box light, and a padded dash board. Production began in March 1964 and official introduction following on April 17 at the 1964 World's Fair.
Several changes to the Mustang occurred at the start of the normal 1965 model year production, five months after its introduction. These cars are known as "late 65's," and were built after factory retooling in August 1964. The engine lineup was changed, with a 200 cu in (3.3 L) "T-code" engine that produced 120 hp (89 kW; 122 PS). Production of the "F-code" 260 cu in (4.3 L) engine ceased when the 1964 model year ended. It was replaced with a new 200 hp (150 kW) "C-code" 289 cu in (4.7 L) engine with a two-barrel carburetor as the base V8. An "A-code" 225 hp (168 kW; 228 PS) four-barrel carbureted version was next in line, followed by the unchanged "Hi-Po" "K-code" 271 hp (202 kW; 275 PS) 289.
Source: Wikipedia (original)