Zenith made a variety of later carbs using the Model-B casting. All of those I have seen without the riveted disc were, judging from components and performance, identical to the typical Model-B carb. Those with the numbered discs riveted to the upper casting above the float bowl were intended for some other use. Most common is the one made for replacement on the Model-A, which features a 20mm venturi and a larger bore #56 inlet valve. The Model-B had a 22mm venturi and #44 inlet valve. However, there is a lot of mixing of parts over the years so you will have to check all of the jets and venturi to determine if this carb will perform well on an "A" or "B" engine. Some of these were designed for industrial or stationary engine use and don't perform very well on a car. The Zenith B-casting sold for replacement on the Model-A forks very well and may even give better gas mileage around town.
