GERMAN WWI RAIDER SS APPAM ARTILLERY SHELL

During WWI in January 1916 the German Raider SMS Mowe captured the British ship the SS Appam. The Appam was sailing out of Senegal to Plymouth England and she was 425 feet long with a gross tonnage of 7,781. The Appam was outfitted to be a Raider and next appeared at Hampton Roads, Virginia on February 1916 and demanded a USA permit to remain in the US indefinitely. This request was denied and the legal fight landed in the Supreme Court ordering the ship to be returned to the British Government. This Shell casing seems to be larger than 20MM and has been mounted on a wooden base. The side of the case is stamped From German Raider Appam 1918. So the question is who made up these displays as it is dated 1918 and should have been back under British control. Was the ship stripped of all offensive weapons by the US? This Shell casing stands 4 1/4” tall with fluted upper leaf. I purchased this item in the early 1970” and it’s been on display all this time. Condition very good ++++