The Motorcraft 2100 & 2150 use different venturi sizes depending on the engine size. The 2 barrel carburetor will usually have the venturi size stamped on the side of the carburetor (Chinese made carburetors may not). This number corresponds to the measurement at (A). This particular carburetor measured 1.21" which would be used on bigger v-8's aka 390 CID.
Video about the venturi size