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 police officers of such cities. Any city which has established a police retirement system under the provisions of sections 86.370 to 86.497 or sections 86.900 to 86.1280 may elect to continue its police retirement system under the provisions of sections 86.900 to 86.1280 even though the city may cease to have the population described in this section, and any city so electing to continue its established police retirement system shall be excused from creating or maintaining any other police 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 “Police Retirement System 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 the effective date of sections 86.370 to 86.497.