B is for "beautiful" if it is a NOS carb, or B for bad, if it is a used one that has been sitting for years. AFB's are great carburetors if they are functioning properly, but can give you lots of headaches when they are not. Relatively easy carburetor to rebuild, if someone else hasn't had their hands in there, and messed with it. Biggest problem with carburetors, are the mass rebuilders of carburetors back 40 years ago. Any part that fit, would be used, even if it wasn't the correct part.