The State plan
(c) Standards. (1) The State plan shall include or provide for the development or adoption of, and contain assurances that the State will continue to develop or adopt, standards which are or will be at least as effective as those promulgated under section 6 of the Act. Indices of the effectiveness of standards and procedures for the development or adoption of standards against which the Assistant Secretary will measure the State plan in determining whether it is approvable are set forth in 1902.4(b). (2) The State plan shall not include standards for products distributed or used in interstate commerce which are different from Federal standards for such products unless such standards are required by compelling local conditions and do not unduly burden interstate commerce. This provision, reflecting section 18(c)(2) of the Act, is interpreted as not being applicable to customized products or parts not normally available on the open market, or to the optional parts or additions to products which are ordinarily available with such optional parts or additions. In situations where section 18(c)(2) is considered applicable, and provision is made for the adoption of product standards, the requirements of section 18(c)(2), as they relate to undue burden on interstate commerce, shall be treated as a condition subsequent in light of the facts and circumstances which may be involved.