The gunsmith Medardus Amrusch, Badgasse 6, Gmunden, is mentioned in the 1950s. During the allied occupation of Austria, from 1945 to 1955, Gmunden, Salzkammergut, Austria, was a center of the United States Forces in Austria (USFA). So the rifle was most likely built for an American then. This explains the lack of any official proofmarks.