The oval cutout gasket is normally used on both carbs despite the form of the front manifold bores. I do believe cadiallic and pontiac used a 4 bore gasket with a cutout for rear anti icing. The thickness of the gasket allow idle air to bleed to the secondaries and frankly acts as a partial communication between primaries and secondary bore. I imagine once throttle blades open the air flow probably can't easily communicate, but it really seems like the idle mixing of idle fuel and air is not ideal as compared to non bypass carbs.