For the last time: Early (and only early) AFB's and WCFB's shared that small bolt pattern.
Chillax. What about this? THE CARBURETOR SHOP / Four barrel mounting flanges
The Carter AFB, the Autolite 4100 and the Holley 4150 were introduced for the 1957 model year. The Carter used the “AFB” mounting pattern, while both the Autolite and the Holley used the “Holley” pattern. The “AFB” pattern measures 4-¼ inch by 5 5/8 inch. This pattern first appeared in the 1957, and continued at least through the 1974 model year for some square-bore carburetors. One issue that MAY be confusing for many enthusiasts is the fact that ON CERTAIN DESIGNS, the front to rear bore centerlines were changed about 1960, with the centerlines being moved further apart by ¼ inch (1/8 inch in each direction from the true front-to-rear centerline). This change was necessary to facilitate the use of larger CFM carburetor designs. For this reason, the throttle plates of some carburetors, even though the mounting flange pattern is the same, will hit the edge of the manifold bores when the plate is fully opened. When installing a later carb on an early O.E. manifold, ALWAYS check for this possibility. Generally, this condition may be corrected through the use of a thicker mounting gasket. Aftermarket manifold manufacturers, aware of the issue, generally produced manifolds with slightly larger diameter throttle bores. Examples of carburetors using the “AFB” pattern:
Carter – WCFB (later production)
Carter – AFB (most)
Carter – AVS (some)
Rochester – 4G series