Thanks to several correspondents
the problem has been solved regarding the strange structures at the Spuikom
Peter Van Onsem was kind enough to send me a post war picture of the crane for servicing German WWII hydroplanes.
During 1947 - 1948 the crane was demolished by contractor Versluys from Bredene. Only 4 socles remain.
During liberation days the Germans tried to destroy the crane but they weren't very successfull.
Updated February 18, 2006, July 19 2009, March 15 2016
Thanks to research from Pierrot
things become more clear now.
This position can be found on German maps as Stp Richthofen.
NGI chart: The structures can be found near the the dot with 'Location'.
Peter Van Onsem send me this picture.
'Kompensierscheiben' to allign the magnetic compass for the airplanes.