31 CFR §337.6

Upon implementation of the book-entry debenture system, to be announced in advance by separate public notice, all new debentures will be issued only in book-entry form, and may not thereafter be converted to certificated form. Certificated debentures may, upon the owner's request in accordance with § 337.5(c), be converted to book-entry. If such action is taken, the owner shall be deemed to have irrevocably waived the right to hold such debenture in certificated form.