It's been over 30 years but I remember 231 as well. Spent my honeymoon in Montgomery and remember, driving from Milton, FL, that we passed a diner/gas station with a big sign on it that said "Eat here and get gas." The fastest route is through Milton. Let me know if the diner still exists.
That's quite a sign, but I think your memory might be fooling you. Milton, FL is about 100 miles west of 231 and definitely not the fastest route to PCB. Milton is next to Pensacola, which my mother constantly mistakes for Panama City.
Interesting note, though, when I-65 terminates at I-10 in Mobile, the guide signs don't display city names like they usually do, but say 'Florida' and 'Mississipp.' Pensacola and Pascagoula would be the cities on the signs, so you can see why they just put the states' names up there.