In all cities that now have or may hereafter attain a population of more than three hundred thousand and less than seven hundred thousand inhabitants according to the last preceding federal decennial census, there are hereby created and established retirement or pension systems for the purpose of providing retirement allowances for civilian employees of police departments of such cities. Any city which has established a civilian employees’ retirement system under the provisions of sections 86.600 to 86.790 or sections 86.1310 to 86.1640 may elect to continue its civilian employees’ retirement system under the provisions of sections 86.1310 to 86.1640 even though the city may cease to have the population described in this section, and any city so electing to continue its established civilian employees’ retirement system shall be excused from creating or maintaining any other civilian employees’ retirement system under any other provisions of the Missouri statutes. Each system shall be under the management of a retirement board to be known as the “Civilian Employees’ Retirement System of the Police Department of (name of city),” and by such name all of its business shall be transacted, and all of its cash and other property held. The retirement systems so created shall begin operation on October 13, 1965, on which date contributions of employees shall be payable to the pension fund.