The present invention pertains to releasable locking mechanism for a hollow circular member such as a fill pipe for an oil tank and cup-shaped closure cap of non-magnetic material which is connectable to the hollow circular member, in combination with a locking member of ferrous material supported by...http://www.google.de/patents/US4223799?utm_source=gb-gplus-sharePatent US4223799 - Releasable locking means for closure caps
