Any employee who is contributing and has contributed to this fund for 20 or more full years, who shall have attained the age of 60 or more years may retire and in lieu of the pension otherwise herein provided, and at his option, receive a monthly pension equal to 1 2/3% of such contributor’s average monthly salary for each full year of service. Provided that for any contributor who retires under the age of 60, who has attained the age of 55 or more years and has contributed to this fund for 20 or more full years, the amount of pension under this section shall be reduced by 1/2 of 1% per month for each month such contributor lacks of attainment of age 60; and provided further that in no case shall the pension exceed 60% of said contributor’s average monthly salary.
     Average monthly salary is defined as the highest average monthly salary for any 5 consecutive years in the last 10 years of service of such contributor.