The carter AVS carbs were just AFBs with a Q-Jet-like Air Velocity Secondary control (adjustible upper butterfly"flap") but on these carbs the diff in primary and secondary butterflie was not so large as Q-Jets, so not near so critical, but these could be adjusted to bring in the sec real smoothly (important to some folks). On early'67 275 hp 327 chevys, some came from factory with these AVS carter AFB looking carbs, the rest had Q-Jets. Since virtually all of GM engines were using the Q-Jet, rochester was having a tough time producing enough to go around. Ultimately GM had to farm out some Q-Jet production to Carter, thus you will sometimes find a Q-Jet with words like "...manufactured for GM by Carter..." on the side of their float bowls. Same carbs
