Briefs as to facts and law will be received and may be required to be submitted in any proceeding. The day for filing briefs will be designated by special order of the Commission during or upon the conclusion of the hearing. Unless jointly agreed to by the parties and the Commission or by order of the Commission, an original and four (4) copies of each brief shall be filed with the Commission and a copy thereof, shall be delivered to or mailed to each party of record prior to or simultaneously with delivery or mailing to the Commission. Certification of such mailing or delivery shall be filed with the Commission.
18- 185 C.M.R. ch. 110, § 21