All files, reports, letters, memoranda, and documents relating to the mediation function of the NMB, with the exception of procedural or administrative materials, such as applications, docket letters, or public meeting notices, in the custody of the NMB or its employees relating to or acquired in their mediatory capacity under the Railway Labor Act are hereby declared to be confidential. No such confidential documents or the material contained therein shall be disclosed to any unauthorized person, or be taken or withdrawn, copied or removed from the custody of the NMB or its employees by any person or by any agent of such person or their representative without the explicit consent of the NMB.