This question is ask quite often here. There should be a sticker on the back of your amp below the speaker jacks. It will tell you if your amp was originally setup for 16 and 8 ohms or 8 and 4 ohms. When you plug into the "Extention" jack, a mechanism on the jack switches the tap on the output transformer to the lower impedance. Despite what it says back there, your amp may have been modified sometime in the last 35 years to the other impedance. But in either case, you will be OK. Another possibility is that the switching mechanism wore out and someone just connected both jacks to a single tap, so to be safe, it's a good idea to look inside to make sure everything is as it should be.
Yellow = 16 ohms
Orange = 8 ohms
Brown = 4 ohms
Black = Ground
