The pic above shows each of the three primary styles of Model B Zenith carburetor upper castings. The top upper casting is the original issue marked 'Z' which uses the large cylinder barrel float. The idle orifice is drilled directly in the cast iron, and the throttle plate has a notch in it and is also marked B1.
The middle casting is the second variety which has a revised idle orifice using a brass fitting. It also uses a round float, though not as cylindrical as the early barrel float. This casting is also marked 'Z' for Zenith.
The lower casting in the pic is the newest and most plentiful upper casting. This design was introduced in Service parts books in 1935, and was used in production in late 1934 per archive research done by David Cockey. It also uses the pressed-in brass idle orifice, but it is not marked Zenith. Instead it has a casting number 32005 on it. This casting has a revised shape around the float valve and uses the rectangular shaped float and baffle.
This pic above shows the three styles in progression. On the left is very early casting for the cylindrical barrel float.
The middle one is the later casting with the brass idle orifice and is used with the round float. Also note the machined clearance for the round float in the roof of the float chamber.
The casting on the right is the typical late style for use with rectangular float.
The pic above shows the three castings in progression. The brass passage plug is evident in the very early style on the left. This plugged the access hole used to drill the idle orifice directly in the opposite wall of the venturi bore.
The castings on both the middle and right are nearly the same except for the shape of the casting and machining in the roof of the float chamber for the round float, and also the float brackets are different between the round float and the rectangular float.
The early cylindrical barrel float bracket on the left has no notch for limiting the float travel. There is also another later variety of the earliest casting on the left which uses the same float bracket as shown on the middle casting. The later round float casting in the middle uses a large notch in the bracket, and the rectangular float bracket on the later casting on the right uses a small notch to control float motion.
This view above shows the differences in the castings in the idle area. The earliest casting on the left does not have the separate brass idle orifice plug. It has the idle circuit drilled from the other side of the bore and plugged on the opposite side as illustrated in the previous pic. Both later castings have the pressed-in orifice plug.
On the casting on the right the brass passage plug in the manifold flange can be seen. This passage was a drill access for the later idle circuit. The middle casting also has this passage plug, though as shown here the brass idle orifice has been pressed out for cleaning the passageways.
Shown above is the upper castings with their respective floats in progression.
Another view of the upper castings in progression. This pic shows somewhat the relative difference between the early cylindrical barrel float on the left, and the subsequent round float in the middle.