Section 13. The Corporation Commission shall in the trial of cases and all special proceedings and hearings admit in evidence only such balance sheets, operating statements and other financial exhibits and schedules as shall have been compiled and authenticated by the treasurer or other regularly acting financial officer or employee of such person, firm, association or corporation as may be a party to the proceeding, action or cause pending before or being tried by said Commission; provided, that if otherwise relevant and admissible, reports of audit, balance sheets, operating statements and financial exhibits and schedules prepared by employees of said Commission, employees of the Office of the Attorney General, and by public accountants as defined in Section 15.1 et seq. of Title 59 of the Oklahoma Statutes or certified public accountants under the laws of the state, shall be admissible and accepted in evidence in all cases.