"The obverse text 'KEINE BANN WARE!', around the upper edge, is translated as 'No contraband goods'. The text in five lines in the obverse exergue 'DER GROSS-DAMPFER=LUSITANIA=DURCH EIN DEUTSCHES TAUCHBOOT VERSENKT 5.MAI 1915' is translated as 'The liner Lusitania sunk by a German submarine 5 May 1915'."
My great uncle, former Buckingham Palace Guard, and then Canadian Infantry, bequeathed this medallion to me. He was given it by my great grandfather.
In shortform, I don't think this mystery is going to be solved.
I have a couple of those in my collection as well.
They were much later proven to be merely a propaganda tool and all issued by the British government, even the supposed “Goetz” version printed in German.
Oddly enough, it was in an article about that very medallion that I first heard of the Lusitania.