It appears to be a German soldier carrying a Soviet submachinegun. Can't quite make out the and at first I thought it was a British Sterling sub gun or 7,65 mm Lindelöf SMG.
I ruled both out because I don't recall a Sterling ever having a wooden stock and the Lindelöf SMG since they most likely never made it into export to Germany. Also it would have been a very low occurence of it having been picked up by a Soviet soldier then the same thing happening by a German soldier.