[Steeveedee]

Looks like Bigdav160 got it. If there is one plug at each exhaust port, they are in the position where the Air Injection Reactor pumped air into the exhaust to help with more complete burning of the exhaust when the choke was engaged. Once the truck warms up enough, the thermal vacuum switch opens and the system freewheels.

That system began to be used in '66 on many vehicles.