A. I f in any prosecution or in any proceeding in respcet of any goods, Burden of the
importation or exportation of which has been prohibited or restricted proof. under section 19,
any question shall arise whether such goods have not been imported or whether no attempt
has been made to export such goods, contrary to such prohibition or restriction, then in such
case the burden of proof thereof shall be on the accused or on the person against whom the
proceedings are opened.