Mo. Rev. Stat. § 52.120
In all counties of the third class in this state that may now or hereafter have a population of twenty-five thousand and less than forty thousand, and in which there is a city of over fifteen thousand population, and in which said city there is a courthouse, more than seven miles distant from the courthouse in the county seat, and in which said courthouse in said city there are held regular and legally established sessions of court of district number 2 of the circuit court of that county, it shall be the duty of the collector of the revenues of such county to maintain in addition to his office at the county seat a branch office in the courthouse located in the said city of fifteen thousand population or more, for the convenience of the taxpayers of said county living within the jurisdiction of said district number 2 of the circuit court of that county.
§ 52.120, RSMo
Effective 1/2/1979