Section 90C. Any city, town or district or the Massachusetts Port Authority which accepts the provision of this section in the manner hereinafter provided may, in the case of a city by two thirds vote of the city council and with the approval of the mayor, or in the case of a town by two thirds vote at the annual town meeting, or in a district by a two thirds vote of its prudential committee or, in the case of the Massachusetts Port Authority, by vote of a majority of its members, increase the retirement allowance of any former employee retired under any provision of this chapter or similar provision of earlier law on account of superannuation after having attained 25 years of creditable service to an amount not exceeding one half the rate of regular compensation payable to employees of such city, town or district holding similar positions at the time of increasing such allowance in the same grade or classification occupied by such former employee at the time of his retirement.