Emerging consensus is that it functioned as a spacer and non-ferrous enclosure, rather than any kind of "blocking" which seems to make sense, but is an oddly disappointing answer somehow.
So, I was watching PBS's Nazi Mega Weapons, and they were discussing the V-1 terror drone when they said something that made me scratch my head. They said that the Nazis initially had problems with the compass that facilitated navigation because of magnetic interference from the metal in the V-1...