CFM was not a value of discussion until the mid to late 1960's, and then only on aftermarket carbs.
Most carburetor manufacturers did not post CFM on O.E. carbs until the late 1970's.
As Buoymaker suggested, the carburetor manufacturers were interested in pleasing the O.E. manufacturers (in this case G.M.)
While both Stromberg and Zenith published CFM charts for engine requirements and for some of their aftermarket carburetors as early as 1930, enthusiasts did not care. Enthusiasts were interested in venturii area.
And while if the folks publishing CFM were honest, CFM would be a more accurate value for comparison than venturi area, as it would take into account differences in air intake (eddy currents), as well as the efficiency of the air flowing through the carburetor; whoever claimed that these folks were honest?
(Does anyone REALLY believe that sales wanted a 600 CFM carburetor, and the engineers experimented with bore diameters and venturii shapes until exactly 600 CFM was flowed)? The numbers were either rounded, are just plain fudged. I know of one company that did not have a carburetor rated at a specific size to compete with another manufacturers carb of the same size, so the sales folks simply TOOK THE NEXT LARGER SIZE AND RENUMBERED AND READVERTISED AT THE LOWER SIZE. Guess which of the manufacturers got the higher HP rates when the carbs were tested by an "independent" laboratory?
Jon.
