A.I.R. = Air Injection Reactor
This is a system that blows fresh air into the exhaust stream via ducts or pipes plumbed into the exhaust manifolds. The purpose of this system is to assist in the more thorough combustion of an overly rich fuel air mixture during deceleration and at high speeds.
To figure out if your truck is so equipped, look for an extra pipe (#1 in the picture) that runs parallel along each exhaust manifold, with 4 separate smaller pipes poking into the manifold at each cylinder. On your V8 powered Chevrolet, there will be the same pipe on each bank of cylinders if it is so equipped. The fresh air is fed to the pipe from a belt driven air pump at the front of the engine with the rest of the belt driven accessories.
I was under the impression that they stopped using this system in the eighties. Having the pipes protrude into the exhaust stream caused unnecessary restriction and turbulence. After the use of aluminum heads became more common and the resulting higher compression ratios used with them, along with advances in ignition systems and of course the more precise fuel/air mixture provided by fuel injection, should make such a system obsolete.