The term “employer,” when used in this article, shall mean the employer as defined in article one of this act,¹ or his duly authorized agent, or his insurer if such insurer has assumed the employer's liability, or the State Workmen's Insurance Fund of this Commonwealth if the employer be insured therein.

The department shall be deemed a “party in interest” in any proceeding under this article before a referee, the board or any court involving any claim for compensation, a part of which is payable by the Commonwealth under the provisions of this act.