Noted another concrete shell illustrated in that Fleischer book,
'The 210mm type 5021 Mortar shell', which appears to be a thin metal (could just be smooth concrete) outer shell with a thick layer of concrete around the central charge. Can't find a picture on the web though.
I see there were also specifically
anticoncrete rounds produced for German guns, some coverage from lone sentry:
"German Anticoncrete Shells -- 150- and 210-mm Calibers" from Tactical and Technical Trends.
Cheers,
Adam.