Fresh air or recirculate vent is not required for AC to work
In fact most vehicles always had a way of closing outside air off way before AC was even offered in vehicles
i.e. Vent on/off or recirculate, as said to prevent unwanted outside air from coming in to cabin, in traffic, smoky, dusty, skunky conditions
Odd the 1983 Ranger didn't have that
When AC started to be used regularly in vehicles auto makers saw a way to bolster THEIR AC over others
"Ours has MAX AC" lol
Most people with AC already knew if they closed the fresh air vent after AC had cooled down the cabin it would get even cooler since AC wasn't having to cool down 95deg outside air, it could recool 75deg air from inside the cabin, they just never named it
Car makers named it MAX AC
AC has two settings, on or off
MAX AC only has to do with vent open or closed, on some models it also shut off heater core circulation, on older vehicles that was done by selecting COLD on the temp slider