All '68-'69 small-blocks with manual transmissions used A.I.R. systems, except those for overseas export with RPO KD1, Less Air Injection. Those units used hex-head 1/4"-18 National Straight Pipe Thread plugs, P/N 3889330, for the holes in the exhaust manifolds.
Sorry for the duplicate threads. Well I'm mistaken, I thought only the 'LOS' built Z/28's had the AIR system and not the NOR built cars? I bought my 68 Z 2nd hand in 1970 and there was no smog equipment on it and the exhaust manifolds had the plugs in them, so now I need to find 68 smog equipment parts, CRAP!
It was common back then to remove the smog system to increase horsepower. Some did it the same day they brought the car home. A buddy bought a 69Z from the original owner, and got several boxes of take-offs in the deal. The smog was removed the first week the guy had the car.