Pictured above is a 1928 choke rod, washer, spring, and retaining sleeve after cadmium plating.
The Model A choke rod consisted of a cadmium plated steel rod which then had an aluminum knob die-cast onto the rod. The knob was not screwed or threaded onto the rod like typical reproductions, and is not removable.
Because the knob is not removable from the rod, the re-plating of the assembly is not straightforward. My method was to clean the aluminum knob, and to bead blast the steel rod first before plating.
Next, dip the knob in the room temperature curing vinyl plastisol sold for use insulating tool handles, like pliers. This material is available in most hardware stores.
Next, the entire rod can go through the cadmium plating operation, and the plastisol will shield the aluminum from the plating solution.
After plating, an exacto knife can be used to slit the plastic and peel it off the knob.
Voila!