Adam, here is some more food for thought....Its possible the extra fitting is on there to meet California emissions. I was just chatting to a guy with a 81 and he says his has 4 fittings on it and one was for the smog pump....Maybe GM just shipped them for either or and blocked off the one not needed.
Swampy

Adam
EFE TVS is Emissions Federal Equip - If you have one of these Thermal Vacuum Switches going to your canister Then Most Likely it is the TR SPK TVS
TR (Thermal Regulated ) SPK (Vacuum Advance) TVS Thermal Vacuum Switch These would both be supplied from a ported vacuum NO vacuum at idle. The one missing would run inline to your vacuum advance This kept the advance from working until the engine reached a predetermined temp a ploy to reduce emissions at cold temps when the choke was engaged.

it's an EGR TVS switch
if you can still see you hose colors it goes like this.
EGR Vacuum hose with purple stripe to bottom port of switch.
EGR Vacuum hose with orange solid stripe on the top port of switch.