Every air carrier shall carry, without charge, on any aircraft that it operates, the following persons:

(a) Security guards who have been assigned to the duty of guarding such aircraft against unlawful seizure, sabotage or other unlawful interference, upon the exhibition of such credentials as may be prescribed by the Administrator of the Federal Aviation Administration;

(b) Safety inspectors of the National Transportation Safety Board or of the Federal Aviation Administration who have been assigned to the duty of inspecting during flight such aircraft or its equipment, route facilities, operational procedures, or airman competency upon the exhibition of credentials or a certificate from the agency involved in authorizing such transportation; and

(c) Postal employees on duty in charge of the mails or traveling to or from such duty, upon the exhibition of the credentials issued by the Postmaster General.