Any fireman who has served in the fire department in the city or who has served in fire departments of one or more cities that come within the terms of this chapter for a period of twenty-five years shall, upon his written application, be retired and receive a monthly pension as provided by this chapter and applicable regulations, but in no event less than one hundred dollars. No fireman shall be required to so apply. This section shall also apply to those firemen retired prior to May 19, 1947, either because of physical disability or otherwise.

Provided, however, that the provisions of this section shall not be construed to expand and obligate the pension system in order to comply with the provisions of this section.