(a) Manner of service. Service upon any party shall be made by the party filing the pleading or document by delivering a copy or mailing a copy to the last known address. If the person upon whom service is made by mail resides 500 miles or more from the party effecting service, such mailing must be by airmail. When a party is represented by an attorney the service may be upon the attorney.

(b) Proof of service. A certificate of the person serving the pleading or other document by personal delivery or by mailing, setting forth the manner of said service shall be proof of the service of the pleading or other document.

(c) Service upon Department, number of copies of pleading or other documents. An original and three copies of all pleadings and other documents shall be filed with the Department of Labor, whether to the Assistant Solicitor in charge of trial litigation, the hearing examiner, or the Assistant Secretary, as the case may be.