After they lost the First World War the Germans were prohibited from manufacturing/selling military arms. To utilize the vast quanties of Mauser rifles in storage, somebody got the bright idea of converting them into shotguns, which would be perfectly legal.
After WWI, germany made up some shotguns using left over military Mausers. modified the bolt to feed and extract shotshells. bolt mod included removing one of the locking lugs. adding the shotgun barrel. the magazine would hold two shells. What you have was originally a Mauser military rifle that was converted to a shotgun