39 USC 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.