In the early years of the Mustang the 'A' designation that you often hear (including myself) represents the factory engine code given to a particular Mustang. The 'A' means a 4 barrel 289 engine. Simple as that. You'll find the engine code in the vehicle's registration or serial number stamped into the inner fenders and on the driver side door tag.
'A' code Mustangs are slightly more desirable because of their performance heritage and lower production numbers as compared to other codes (C code- 2 barrel 289, T code - 200 cid) but not as desirable as the factory K code which boasts a 65-66 Mustang high of 271HP.
'65 and '66 Mustangs had a V emblim on the front fenders just behind the headlight
if they came with a V8.
On early '65 ('64 1/2) the base V8 was 260. The next level V8 was the 289 4V 9:1 comp
rated at 210 hp. Later in the '65 model came the 289 2v 200 hp, 289 4v 10:1 comp
225 hp and the 289 4v high performance 271 hp. The 260 and 289 showed the size
with the V. The high performance (k code)had a high perfomance emblem above the 289.