Z28 LT1 valve covers

I have a pair of OEM Z28 LT1 finned valve covers with oil drippers that I massaged and polished to make them look better on my 355. They are not perfect but they still look good and they serve a purpose. Then I bought the discontinued GM Performance catalog version of these same valve covers just to have them. The supply seems to be drying up and I can always sell them on eBay where you can still buy them. There are some differences on the underside. Does anyone know if the originals or any other version were made from anything other than aluminum? Maybe magnesium? I have my reasons to think so but I will not bore you with the details.