The following pics from Ford parts catalogs show the Model B Ford Zenith carburetor service parts.
The original 1932-34 Model B and Model 46 vehicles all used the early style carburetor with the round float assembly.
Ford changed to the rectangular float design in late 1934 for service and regular production.
After production ended they offered the rectangular float assembly, spring, axle, baffle, and bracket as a service part for early carburetors needing a replacement float. Round floats were discontinued after 1935 through Service.
These carburetor differences affect both the upper and lower castings, the throttle plate, the float bracket, the idle passage, and more.
This is the typical Ford parts book lower body illustration used throughout the 1930s and 1940s. The actual lower bodies used with the round and rectangular floats are somewhat different, though not really evident in this 1936 illustration.
This same illustration is also in the June 1935 catalog. The rectangular float chamber has a 'shelf' on the inside wall of the lower casting.
Most round float carburetors do not have any shelf in side the lower casting wall.
There are however, some early round float carburetors which have a shelf with an oval notch in it for float swing clearance. These appear to be a transition carburetor made during the conversion from round float to rectangular float production.
Above is the parts book illustration from July 1936. This same illustration is also in the June 1935 catalog.
Although the B-9550 round float is shown, it is obsoleted at that time and noted to be serviced with the B-9652-R assembly, which consists of the rectangular float, spring, axle, baffle, and bracket.
The rectangular float assembly is noted in the Parts Catalog footnote as providing improved fuel economy.
The mounting brackets are different between the two floats. The rectangular float bracket has more restricted movement than the round float bracket.