12 U.S.C. § 2291

In order to furnish obligations for delivery by the Bank, the Secretary of the Treasury is authorized to prepare such obligations in such form as the Bank may approve, such obligations when prepared to be held in the Treasury subject to delivery upon order by the Bank. The engraved plates, dies, bed pieces, and other material executed in connection therewith, shall remain in the custody of the Secretary of the Treasury. The Bank shall reimburse the Secretary of the Treasury for any expenditures made in preparation, custody, and delivery of such obligations.