Can't remember the source of supply for the ultrasonic cleaner.
The castings are a mixture of various metals, zinc is among them, lead as well.
The carburetors were given a dip in chromic acid from the factory. This impregnates the castings and reacts with the lead and zinc. Since very early carburetors contained more lead in the material, they typically come out darker in appearance.
I still use chromic acid here, there are several other methods to "color" the castings, but I much prefer the original type finish.
We turn recoloring jobs around quickly, typically in about a week.....Cliff