(a) When books, records, papers, or documents have been received in evidence, a true copy thereof or of such part thereof as may be material or relevant may be substituted therefor, during the hearing or at the conclusion thereof.

(b) After a decision has become final, upon request and after notice to the other party, the Board in its discretion may permit the withdrawal of original exhibits, or any part thereof, by the party entitled thereto. The substitution of true copies of exhibits or any part thereof may be required by the Board in its discretion as a condition of granting permission for such withdrawal.