Except as modified by the provisions of Sections 12-133.1 and 12-133.2, the maximum retirement annuity for any employee under the provisions of this Article shall be 70% of the highest average annual salary for any 5 consecutive years within the last 10 years immediately preceding the date of withdrawal; provided that in the case of an employee in service on June 30, 1957, the maximum retirement annuity shall be the amount prescribed by the provisions of “The 1919 Act” in effect on June 30, 1957, increased by the amount resulting from accumulations accruing during service rendered thereafter consisting of contributions by the employee and employer for service annuity, improved by regular interest, subject to a maximum annuity equal to 75% of the highest salary received by an employee while in service for that part of such salary which does not exceed $6,000 per year, and 60% of that part of such salary which exceeds $6,000 per year.