Each contract market shall require that all contracts of sale of any commodity for future delivery on or subject to the rules of such contract market shall provide for the delivery thereunder of commodities of grades conforming to United States standards if such standards shall have been officially promulgated and adopted by the Commission. In the event of a change in United States standards, all contracts made on and after the effective date of the adoption of the revised standard by the Commission shall be made on the basis of the standards as changed: Provided, That this shall not be construed to prevent the closing of trades made prior to the effective date of such adoption by the Commission.