You have experienced one of the many problems with the pre-1967 Rochester carburetors. Prior to 1967, Rochester carburetors were identified ONLY by tag. As the commercial carburetor rebuilders "group" carburetors of different years and applications into a common group, they remove the tag such that the customer has no idea which carburetor was sold to them.
As to your specific problem: Rochester used many DIFFERENT accelerator pumps in the type B carbs over the years. These vary in design (flat, and round) length, and material (leather, and neopreme - leather is much superior). I have not seen a variance in diameter, but I wouldn't bet against it.
Jon.
