"Back in the day" as they say, I used to buy a 'matched set' of non-cogged belts for my 61 cars with A/C from NAPA (since two are required). It has been many years since I've seen such matched non-cogged sets. The same NAPA store I used to go to told me quite a few years ago that the manufacturing tolerances had become so good that there was no longer a need for a 'matched set' of belts, as they are all so close to the exact same length individually.
I seem to remember a belt number like 15680 for the alternator w A/C, and I think the '68' part of the number designated a belt length of 68 inches, but my memory might be playing tricks on me.