There's a #ecz-9424-A for sale on ebay, what's the difference bergen the #ecz-9424-B ?

The ECZ-9424-A is a 4 bbl manifold. It has a square mounting bolt pattern for 55 and 56. This manifold typically came with the teapot carb used on Ford and Thunderbirds.  Some 56 Mercs used a Carter WCF? that has the square bolt pattern.
Starting in 1957 The 'B" manifold was used. The mounting pattern was larger and rectangular instead of square. Many different 4 bbl carbs will mount on this manifold - Bob

AKA   Bob-93021