The Zenith-3 came out in January 1931; the 1st version had the gas inlet at the front of the upper housing like the Zenith 1 and Zenith-2; the mounting to intake manifold is very thin and has two rienforcing gussets for strength. In May the Zenith-3 Side Bowl was introduced, it used the same lower bowl, only the upper housing gas inlet was changed to accomodate the side bowl. The Zenith-1 and Zenith-2 were also modified to accept the side bowl.
Ron