Sec. 21.603 - TSO marking and
(a) Except as provided in paragraph (b) of this section and §21.617(c), no person may identify an article with a TSO marking unless that person holds a TSO authorization and the article meets applicable TSO performance standards.
(b) The holder of an FAA letter of acceptance of a statement of conformance issued for an article before July 1, 1962, or any TSO authorization issued after July 1, 1962, may continue to manufacture that article without obtaining a new TSO authorization but shall comply with the requirements of §§21.3, 21.607 through 21.615, 21.619, and 21.621.
(c) Notwithstanding paragraphs (a) and (b) of this section, after August 6, 1976, no person may identify or mark an article with any of the following TSO numbers:
(1) TSO-C18, -C18a, -C18b, -C18c.
(4) TSO-C61 or C61a.