Upon retirement from service as provided in Section 21-29-31 a member shall receive a service retirement allowance for the remainder of his life, payable monthly, which allowance shall be a sum equal to fifty percent (50%) of his average monthly salary for the last four (4) years of service immediately prior to retirement; provided, however, that upon retirement from service in any municipality having a commission form of government on March 31, 1948, as provided in Section 21-29-5 and having a separate firemen and policemen retirement system and having a population not in excess of 48,000 according to the 1980 decennial census, which municipality has established a retirement system according to the provisions of this chapter, the service retirement allowance shall be based upon the average monthly salary for the last two (2) years of service immediately prior to retirement. Any member who has been in paid service for longer than twenty (20) years shall be entitled to receive additional payments for life in a sum equal to one and seven-tenths percent (1.7%) of the average monthly salary received by such member in the two-year period next preceding the filing of said application for each full year of service in excess of twenty (20) years’ service. However, no retired payment to any member shall exceed sixty-six and two-thirds percent (66-⅔%) of the average monthly salary received by a member for the four-year or two-year period, as the case may be, next preceding the filing of said application. Periods of time in which a member may have been inactive on account of physical or mental disability shall not be excluded in computing the twenty-year period and the seven-year period hereinbefore mentioned, provided the employee continues to contribute his share to the program based on the average of his last two (2) months’ compensation. This contribution shall continue during the total time the employee shall be inactive.