§ 57. Continuous recruitment for certain positions. Notwithstanding any other provisions of this chapter or any other law, the civil service department or a municipal commission may establish a continuing eligible list for any class of positions for which it finds inadequate numbers of well qualified persons available for recruitment. Names of eligibles shall be inserted in such list from time to time as applicants are tested and found qualified in examinations held at such intervals as may be prescribed by the civil service department or municipal commission having jurisdiction. Such successive examinations shall, so far as practicable, be constructed and rated so as to be equivalent tests of the merit and fitness of candidates. The name of any candidate who passes any such examination and who is otherwise qualified shall be placed on the continuing eligible list in the rank corresponding to his final rating on such examination. The period of eligibility of successful candidates for certification and appointment from such continuing eligible list, as a result of any such examination, shall be fixed by the civil service department or municipal commission but, except as a list may reach an announced terminal date, such period shall not be less than one year; nor shall such period of eligibility exceed four years. Subject to such conditions and limitations as the civil service department or municipal commission may prescribe, a candidate may take more than one such examination; provided, however, that no such candidate shall be certified simultaneously with more than one rank on the continuing eligible list. With respect to any candidate who applies for and is granted additional credit in any such examination as a disabled or non-disabled veteran, and for the limited purpose of granting such additional credit, the eligible list shall be deemed to be established on the date on which his name is added thereto.