The interpretations of the law contained in this part are official interpretations which may be relied upon as provided in section 10 of the Portal-to-Portal Act of 1947. In addition, the Supreme Court has recognized that such interpretations of the Act “provide a practical guide to employers and employees as to how the office representing the public interest in its enforcement will seek to apply it” and “constitute a body of experience and informed judgment to which courts and litigants may properly resort for guidance.” Further, as stated by the Court: “Good administration of the Act and good judicial administration alike require that the standards of public enforcement and those for determining private rights shall be at variance only where justified by very good reasons.” (Skidmore v. Swift, 323 U.S. 134.) Some of the interpretations in subpart D of this part relating to the scope of the exemption provided for retail or service establishments are interpretations of this exemption as it appeared in the original Act before amendment in 1949 and 1961, which have remained unchanged because they were consistent with the amendments. These interpretations may be said to have Congressional sanction because “When Congress amended the Act in 1949 it provided that pre-1949 rulings and interpretations by the Administrator should remain in effect unless inconsistent with the statute as amended. 63 Stat. 920.” (Mitchell v. Kentucky Finance Co., 359 U.S. 290.)