39 U.S. Code § 3203. Endorsements on penalty covers

(a)
Except as otherwise provided in this section, penalty covers shall bear, over the words “Official Business” an endorsement showing the name of the department, bureau, or office from which, or officer from whom, it is transmitted. The penalty for the unlawful use of all penalty covers shall be printed thereon.
(b)
The Postal Service shall prescribe the endorsement to be placed on covers mailed under clauses (1)(E), (2), and (3) of section 3202(a) of this title.
(Pub. L. 91–375, Aug. 12, 1970, 84 Stat. 752.)
Effective Date

Section effective July 1, 1971, 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.