The President may not enter into a bilateral or multilateral agreement authorized by subsection (a) of this section unless the application of the import restrictions set forth in section
2606 of this title with respect to archaeological or ethnological material of the State Party making a request to the United States under article 9 of the Convention will be applied in concert with similar restrictions implemented, or to be implemented, by those nations (whether or not State Parties) individually having a significant import trade in such material.