I agree with everything said.If you've got the style that was used with the B engine. There was a service replacement B carb that was sold as the first replacement for the Zenith one two and three carbs, this carb had a smaller inside diameter venturi. The larger B venturi will interchange. The B intake has larger passages and I think that the flat top of the B intake may have been for better flow and distribution to the cylinders.The Swan design intake manifolds were flat across the length. Here is a picture of the intake and dual B carb set up on our speedster. The name on the intake is UNI-FLO but it looks like the swan design. It was made in the thirties for the B engine. It has large ports and runners and way out performs the Trojan dual up draft manifold and B carbs on our roadster.