Proof of service of any notice, order, or other document issued by, or under the authority of, the board may be made in the same manner as in the service of a summons in a civil action. Proof of service shall be filed with the board or may be made by forwarding a copy of that notice, order, or other document by registered mail, directed to the person at his last known address, with an affidavit to that effect being filed with the board.