Officers of district land offices designated by the Secretary of the Interior in all matters requiring a hearing before them are authorized and empowered to issue subpoenas directing the attendance of witnesses, which subpoenas may be served by any person by delivering a true copy thereof to such witness, and when served, witnesses shall be required to attend in obedience thereto: Provided, That if any subpoena be served under the provisions of this section by any person other than an officer authorized by the laws of the United States, or of the State or Territory in which the depositions are taken, the service thereof shall be proved by the affidavit of the person serving the same: Provided further, That said subpoenas shall be served within the county in which attendance is required, and at least five days before attendance is required.