A couple comments. The magnesium case halves were cast by VW for Porsche, so they have all the normal VW markings and part numbers on them, including the date cast into the end of each case half in the area covered up by the intermediate plate. From your photo.
I can see the casting dates are 6-Sep-1955 and 5-?-1955. The format is (DD)(MM)(SY) where S=shift 1,2 or 3, and Y=year in 195_.
Also, I agree with Mike Smith about protecting the case from corrosion. After vapour blasting that case, it has no protection on it now, and you will absolutely want to put something on it. Otherwise it will turn to dust. Literally. Mike's suggestion is good, or Henkel makes an Alodine magnesium conversion coating that you can spray or brush on and rinse off. It should really be done before assembly on just the case, but certainly doing it now is better than nothing.
You should also do your black oxide (or zinc oxide) coating on the intermediate plate, as that too will rust on the outer surface exposed to the road.
Great thread and great photos!