RULE 51A. Removal of goods after payment of DutyOtherwise expressly provided in these Rules, no duty-paid goods shall be allowed to enter, or be retained in, any part or premises of a factory: provided that the Commissioner or the Board may, by a general or special order, and subject to such conditions and limitations as may be laid down in such order, permit duty-paid goods to enter, or to be retained in, any part or premises of a factory.