20 x 110 Hispano by F.Co


I have an empty 20x110 Hispano 404 case with the headstamp “F.CO. 20mm 1942” I have tentavily ID’ed the maker as Frost Company, Kenosha, Wisconsin. If I have the ID correct, then I have a question. The case has no primer, but the primer pocket is a Berdan type. If this is a U.S. made case, why the Berdan primer?