39 U.S.C. § 3207
(a) Penalty mail is restricted to articles not in excess of the weight and size prescribed for that class of mail receiving high priority in handling and delivery, except—
(b) A penalty mail article which is—
is mailable at rates for that class of mail entitled to the lowest priority in handling and delivery, even though it may include written matter and may be sealed.
(Pub. L. 91–375, , 84 Stat. 753.)
Section effective , pursuant to Resolution No. 71–9 of the Board of Governors. See section 15(a) of Pub. L. 91–375, set out as a note preceding section 101 of this title.