While developing the C-3PO Collectors Case for the Original Collection, Kenner designers experimented with vacuum metalled (AKA: Vac-Metal) finishes on Darth Vader cases. They used both full production cases as well as new castings utilizing different coloured plastics, all in the interest of producing a high-quality product. Once they started nailing it down, Kenner began shipping these prototypes around the country as a final test to see how well the finish would keep. Follow this link to check out the Vac-Metal Darth Vader Case
Do note, there are a number of fakes out there, so if you are interested in this piece, make sure to research it further.