There was a very good thread posted a few weeks ago about the hardrive cases.
The black painted Ali part is actually 'Extrusion', can't remember the number code though.
Its a chunk of extrusion, heated up hot, then placed in the die and pounded into shape in one hit.
I rearranged my Ali.sacks to take this into account a few days ago.
The silvery Ali floppy drive chassis is another Ali grade, the post tels the actual number, its al different from the real 'Cast Ali' grade too. Like the 'cast in a sand mold Ali grade'.
I tend to find that both Ali and Stainless get scratched by the edge of a screwdriver, but on Ali the scratch has a very slight burr on its edge.
Those hard drive cases/covers, the silvery part, is sometimes a sheet of Stainless glued to the Ali.