I think it was late '28 when Ford changed the terminal box from Fordite compound (bakelite type material) to hard rubber. Knowing that may be useful in your decision.
I use warm water and soap, and they come out nice. If they have been out in the open and are quite weathered to a rough texture I will use a fine sandpaper and lightly go over the finish to smooth it some. I've never cleaned them while they are mounted on the car.
Have you tried a firm-bristle toothbrush using dishwashing soak (Dawn, etc.) and water to clean it, ...and then use the same toothbrush with polishing compound to do the final clean-up/polishing? I find it works well.
Along the same lines as cleaning these parts, what is a good way to store them? I have several bakelite and hard rubber parts that have been cleaned and are stored in the basement. After a couple years I see mold spots on them.