The belts have production codes/batch codes cast into them when made. Some might be silk screened on, but there is notation of which "batch" they came from. So "matching" can still happen at the store level, IF they have enough of the needed belts in stock to do that.
I concur that modern belts are made more accurately, so matching as it used to be, might not be as necessary as back then.
Your mechanic might perceive he has your best financial interests at heart, but it can also be that he does not have a R-12 recovery machine any more. To use his existing R-134a machine would contaminate it, so best to not get anywhere near anything with R-12 in it. Unless you might want to buy a refurbished R-134a recovery machine for him.
It would be FAR easier to just leave the a/c compressor installed and unplugged electrically than to try to remove it from the existing set-up. Which does not address the different pulley ratios of the factory a/c vs non-a/c belt drives.
In the world of v-belts, most OEMs used two different widths of belts. .380" and .440" wide. The .380" belts sit lower in the pulley grooves, as they are narrower, while the .440" belts are wider and sit higher in the same grooves, which also means then need to be about an inch longer in length compared to the narrower .380" belts. This is not "brand specific", though.
CBODY67