(a) If service of the subpena is made by a person other than a United States marshal or his deputy such person shall make affidavit thereof, stating the date, time, and manner of service.

(b) In case of failure to make service, the reasons for the failure shall be stated on the original subpena by the person who attempted to make service.

(c) The original subpena, bearing or accompanied by the required return affidavit or statement, shall be returned forthwith to the Secretary of the Commission or, if so directed on the subpena, to the official before whom the person named in the subpena is required to appear.