The following pics from the Zenith Corporation parts catalogs show some interesting information about Model B Ford style Zenith carburetors and service parts, as well as the Model A replacement carburetor (B style) offered through Zenith dealers (different than offered by Ford Service).
This is the Zenith parts book illustration showing the two models, one for Model B's and one especially for Model A's. The Model A version had a smaller venturi size, a different float valve for gravity feed rather than pressure, and smaller main and cap jetting.
These carburetors were the 124-1/2 series. It is not clear why they are labeled as 1" size, since Ford described them as 1-1/8" size, and the throttle bore actually measures 1-1/4".
This series was started by the carburetor originally provided to Ford for the Model B, and went on to include several other similar carburetors with balance tubes, vacuum economizers, accelerator pumps, and other improvements, and were found on many OEM industrial and agricultural applications throughout the 1930s.
The Model B and A versions were the only ones to use the familiar Ford choke control and GAV.
This is the Zenith parts book illustration showing the kit made available to Zenith dealers expressly for modifying the Model B and A (replacement) carburetors for customers.
The chart above shows the Zenith number for the venturi, jets, and valves specified for both the Model A and B version carburetors. This chart mistakenly identifies the Model A stroke length as 4-1/2 inches. It is actually 4-1/4 inches.
Zenith used the millimeter unit to designate sizes. For example, the Model B used a number 22 venturi. This means it was 22 mm inside diameter, which is equal to 0.866 inches. Model B venturis are found marked either "22" or ".866".
For jet numbering, each unit of jet size equals 0.05 mm. Thus a number 20 main jet has an inside diameter of (20)*(0.05) = 1.0 mm, which is equal to 0.039 inches.
The standard Model B carburetor float valve is marked "44" and was for use with a fuel pump, whereas the Model B valve for aftermarket Model A applications was marked "54" for use with gravity feed.
The "54" float valve had a larger diameter orifice than the standard "44" Model B valve.
The chart above shows the amount of airflow at different vacuum levels for the different venturi sizes. The Model B style Zenith was a 124-1/2 series carb, and the Model B version used a number 22 venturi. The Model A version used a number 20 venturi.
This chart column above is mistakenly labeled 124-1/4 rather than 124-1/2. The 41" refers to manifold vacuum in inches of water, and 41" is the proper column to use for this application according to the text in the manual.
The chart above shows the amount of air consumption for different displacement engines at different RPMs. The Model A and B are both 200.5 Cubic Inch Displacement stock, and 214 CID at 0.125 inch oversize bore.