This is "Diff Dunker" differential cleaning tool from GMK Supply. The "Diff Dunker" diff cleaning tool, when used with naptha, makes cleaning differential parts a snap. You put the differential parts - gears, pins, outdrives, etc. into the Diff Dunker, swirl it around in a jar with naptha in it for 20 - 30 seconds, and then empty the parts onto a paper towel or rag. The naptha evaporates quickly, and the diff parts are clean and free of any silicone or dirt.
Naptha VM&P is available at almost any paint or hardware store in the paint section. Includes directions on how to use.