Mo. Rev. Stat. § 51.121: County clerk to inspect precincts, report required, when required.

In any county of the first class not having a charter form of government, and all counties of the second, third or fourth class, the county clerk shall annually, on or before May tenth, inspect all voting precincts in the county, review the described boundary lines, and survey the number of voters in each precinct measured by the vote at the last preceding presidential election, and within thirty days after the conclusion of such inspection, present a signed report to the county commission and the county chairman of the two political parties receiving the largest number of votes in the last presidential election, detailing changes, alterations, and additions which appear to be necessary for the convenience of the voters.