Ok I just had to go look at the 64 AFB on my 65 Corvette. It does not have ported vacuum. Yes I know it's not a mopar but what chevy does is use manifold vacuum to increase timing under idle and light loads as a way to run more advance for better cruising and better gas mileage plus allowing for more mechanical advance.
My factory distributor has 22 degrees mechanical that comes in early and when you add vacuum under low loads you have at least 35 total advance and closer to 40 cruising. But when you accelerate fast you lose vacuum and therefore retard timing so the engine doesn't knock.

Just the opposite of Mopar.