It's not at all unusual for there to be 'extra' vacuum ports. Some of the ports may have a little more vacuum than others and may work better/worse for the vacuum advance. On some original cars, some vacuum lines went to the windshield wipers and other accessories. Worse case is you need one for the vac advance and PVC if you have a PVC port in your valve cleaner. If not, keep it plugged too, adding a PVC won't really help the engine run any better, it just pulls air from inside the valve cover and let's it go in the engine with the fuel mixture. If your car doesn't have one, you probably have a vented oil filler cap on the valve cover and you may see smoke coming from it once in a while...not a problem. Thing to remember on vacuum ports is to use it or plug it. You DON'T want any leaking vacuum lines or the car will run crappy.
