Section 100A. If, before an award is determined upon by the referees, any referee, including a referee appointed under this section, dies, resigns, is incapacitated, removes from the commonwealth or for any other reason is unable or refuses to serve, the company, if such referee was chosen by the insured, or the insured, if such referee was chosen by the company, or the company, the insured or the two referees chosen by the insured and the company, if such referee is a third referee chosen by the said two referees, or the company, the insured or either of said two referees, if such referee is the third referee appointed by the commissioner, shall forthwith make written application on oath to the commissioner in such form as he may prescribe for the appointment of another referee. The application, unless it seeks the appointment of a third referee to succeed a third referee appointed by the commissioner, shall specify the full names and addresses of three persons. The commissioner shall, after such summary inquiry or hearing, if any, as he may deem expedient, appoint a referee to fill the vacancy, but if the application specifies names as aforesaid, he shall appoint one of the persons so specified. The commissioner shall give written notice of the appointment to the appointee, to the parties and to the other referees. Nothing in this section shall be construed to prohibit the insured and the company from filling any vacancy by mutual agreement.