16. All pensions or retirement allowances granted under the provisions of this act shall be effective on the first day following retirement, shall be paid in equal monthly installments, except that in those instances where payment is required for a portion of a month which is not complete a pro rata payment shall be made, and shall not be decreased, increased, revoked or repealed, except as otherwise provided in this act.
Upon the death of a retirant, any unpaid benefits due him shall be paid in one lump sum to his beneficiary. No pension or retirement allowance shall be due to a retirant or a beneficiary unless it constitutes a payment for an entire month; provided, however, that a pension, annuity or retirement allowance shall be payable for the entire month in which the retirant or beneficiary dies.