Super informative. I'm going to give the 525 belt a go and see. I compared the old with the 520 today and the difference is minimal but obviously enough to make a big difference. I just got it back from having leaky manifold gasket replaced so these are the the only things I haven't replaced. I did look at that video as well and as you correctly stated I have the A.I.R pump on its own belt. I will note all the belt sizes once I have them correct for future reference. New to the Oldsmobile scene but I got a super nice car here so just want it correct. Thanks again.Your 86 Cutlass VIN Y should be this belt-
9433757 7/16 x 52.5" - This A/C belt was used on the single belt P.S. pump setups along with the single belt from water pump to alternator setups for 307 Cutlass. (see the video- it shows a similar setup). You can tell easily by the single groove P.S. pump pulley and 2 different sized water pump pulley grooves.
9433756 7/16 x 52.0" - This A/C belt is called out for the DUAL belt P.S. pump setups with part of the alternator belt running around the front groove of the P.S. pump. This setup was used on SOME Cutlasses mid-84 production and up, mostly VIN9, and IIRC, the Diesels.
Some aftermarket belts come with the length size in their part number. Examples you gave was the 520 and 525 belts. 520 equates to 52.0" and the 525 equates to 52.5".
Obviously it doesn't have to be a GM belt, I'm just throwing the p/n's out there as to what the sizes of the belt are. Pick the one best for you, and you should be golden.
Edit: I went back and looked at the video and I thought I saw that A/C belt number was 15585. I'm like "dafuq"?? That seems weird. Then I realized it was likely a Custom Cruiser setup with the A/C belt driving the A.I.R. pump too. Errrgh. 86 Cutlass drove the A.I.R. pump via an extension pulley off the crank specifically for this, as did many others. Olds used about 43 different belts in the 80s and early 90s on the 307s.