If the Commission finds, on the basis of an investigation under subdivision (a) of this section, that in any calendar year after 1938 the quantity of imported red cedar shingles entered for consumption, or withdrawn from warehouse for consumption, was in excess of 30 per centum of the combined total for such year of the respective quantities ascertained in such investigation, it shall so report to the President. If the President approves the report of the Commission, he shall so proclaim, and on and after the day following the filing of such proclamation with the Division of the Federal Register and so long as any trade agreement entered into under the authority of section
of this title, shall be in effect with respect to the importation into the United States of red cedar shingles, there shall be a duty upon imported red cedar shingles entered for consumption, or withdrawn from warehouse for consumption, in any calendar year in excess of 30 per centum of the annual average for the preceding three calendar years of the combined total of the quantity of such shingles shipped by producers in the United States and of the quantity of such imported shingles entered for consumption, or withdrawn from warehouse for consumption. The rate of such duty shall be 25 cents per square. Any duty imposed under this section shall be treated for the purposes of all provisions of law relating to customs revenue as a duty imposed by section
1001 of this title, and shall not apply to shingles entered for consumption before the duty becomes applicable.