Section 91B. Constables shall not be appointed by mayors or selectmen under section ninety-one or ninety-one A except as hereinafter provided. A person desiring to be appointed as aforesaid shall make a written application therefor to the appointing authority stating his reasons for desiring such appointment and such information as may be reasonably required by said authority relative to his fitness for said office. Such application shall also contain a statement as to the moral character of the applicant signed by at least five reputable citizens of the city or town of his residence, one of whom shall be an attorney-at-law. The appointing authority shall also investigate the reputation and character of every applicant and his fitness for said office. The chief of police or other official having charge of the police shall upon request give the appointing authority all possible assistance in making such investigation. The office of constable shall be filled only by appointment of an applicant hereunder who is found by the appointing authority, after investigation as aforesaid, to be a person of good repute and character and qualified to hold said office.