Not to argue with the expert, BUT AVS stands for Air Valve Secondary.
I have a '70 small primary on my 292 and a '69 383 small primary bagged on a shelf. They both have the "AVS" cast on the drivers side, rear.
I think the AVS is a great carb and the Air Valve spring allows you to adjust when YOU want the secondary to come in, by simply tightening/loosening the spring. The secondary boosters above the weighted secondary valve on the AFB caused a delay in fuel delivery. Carter moved the air valve above the fuel discharge and solved two problems.
I also have to say my Rochester B worked very well. It was just too small and looked funny on the Clifford intake.

'67 GMC 3/4 292 4spd