Okla. Stat. tit. 19, § 297

It is hereby made the duty of county clerks of Oklahoma to record without charge all certificates describing and designating lands situate in their respective counties owned by members of the Five Civilized Tribes and their heirs, which lands are shown by said certificates to be exempt from taxation under the provisions of Section 4, of the Act of Congress, approved May 10, 1928, Public Number 360 - 70th Congress; provided, said certificates when presented for recording bear the approval of the Secretary of the Interior.