Annuitants – Retirement from service – Notice – Amount of pension. Any person who has been an employe for a period of 20 years or more who shall have attained the age of 50 years, and is a contributor to said fund, and shall have contributed to said fund for the same period, may retire upon 60 days’ notice to be given to said board of trustees unless such notice is waived by said board of trustees, and become an annuitant under this act and be entitled to a monthly pension of $60. Provided, that for every additional full year of service after 20 years rendered before such retirement, an increase of $7 per month shall be added to the monthly pension allowed until the maximum of $200 per month shall have been attained; And, provided further, that no pension shall exceed 60% of the maximum annual salary received during the employe’s term of service.