Section 77C. In any city or town to which section seventy-seven applies and which accepts this section, in the manner hereinafter provided, a laborer who was in the employ of any department of such city or town prior to July first, nineteen hundred and thirty-seven, and on or after said date was promoted to a supervisory position in the same department, shall not by reason of the acceptance of such promotion be deemed to have forfeited his right, if any, to a non-contributory pension under the provisions of said section seventy-seven.
This section shall take effect in a city having a plan D or plan E charter by the affirmative vote of two thirds of all the members of the city council, and in the case of any other city by vote of the city council subject to the provisions of its charter, and in a town by a majority vote at an annual town meeting.