A case for the German administrators of the site:
The "Nartum Steingrab" is a long barrow and not a chambered tomb. The relevant literature from the offical responsible archologist Dr. Tempel describes the large stone grave as a long barrow.
You can check this information here:
Tempel, Wolf-Dieter (1999): Auf ur- und frühgeschichtlichen Spuren durch den Landkreis Rotenburg, Wegweiser zur Vor- und Frühgeschichte Niedersachsen - Heft 22, Oldenburg p. 42-43
Something is not right. This message is just to keep things from messing up down the road