Section 45B. In any city or town which has accepted sections forty-four and forty-five A in the manner set forth in section forty-five and which accepts this section, a school janitor or school custodian who was in the employ of the school department of such city or town prior to July first, nineteen hundred and thirty-seven, and on or after said date has been, or hereafter shall be, promoted to a supervisory position in the janitorial or custodial service of such school department, shall not by reason of the acceptance of such promotion forfeit his right, if any, to a non-contributory pension under the provisions of said sections forty-four and forty-five A.