Section 74. Any woman who entered the continuous employ of the superintendent of buildings or the sergeant-at-arms as a cleaner and scrubwoman prior to July first, nineteen hundred and twenty-one, if she has reached the age of sixty and has been in such employ for not less than fifteen years and has become physically or mentally incapacitated for labor, or any such employee if she has been in such employ for not less than ten years and has become physically or mentally incapacitated for labor by reason of any injury received in the performance of her duties, may, with the approval of the superintendent, be retired from service, and if so retired she shall receive from the commonwealth for the remainder of her life the sum of three dollars per week.