Upon request of any party showing good cause therefor, at the prehearing conference or otherwise upon notice to all other parties, the Administration or presiding officer may direct any party to produce and permit the inspection and copying or photographing, by or on behalf of the moving party, of any designated documents, papers, books, accounts, letters, photographs, objects, or tangible things, not privileged which constitute or contain evidence relating to any matter, not privileged, which is relevant to the subject matter involved in the pending proceeding, and which are in his possession, custody or control. The order shall specify the time, place, and manner of making the inspection and taking the copies and photographs and may prescribe such terms and conditions as are just. In lieu of such inspections the material may be produced in exhibit form and served upon all parties to a formal proceeding. Such exhibits may also, upon request of any party, be offered in evidence at a hearing.