An authorisation given under Rule 56 by a dealer or other person to appear on his behalf in any proceedings before any proceedings before any Sales Tax Authority, other than the High Court, shall continue to be valid for the purpose of appearance in an appeal against, or application for revision of, any order passed by such authority:

Provided that a separate authorisation shall be furnished for appearance in proceedings relating to each period for which a separate order of assessment is required to be made or has been made under the Act.