The prohibition against imports in, and the penalty of forfeiture of, certain vessels from countries which discriminate against American vessels provided for in subsections 2 and 3 of paragraph J, section IV, Tariff Act of 1913, as amended by the act of March 4, 1915 (19 U.S.C. 130, 131), shall be enforced only in pursuance of specific instructions issued and published from time to time by the Secretary of the Treasury or such other officer as the Secretary may designate. (See also §§ 4.20(c) and 159.42 of this chapter.)