Mo. Rev. Stat. § 445.050: When new county established or county lines changed, plat to be recorded, where.

When any such maps or plats shall be recorded, and by reason of the establishment of any new county, or the alteration of any new county lines, the city, town or village shall fall within a different county, such recorder shall transmit a certified copy of such plat or map to the recorder of the county, within which such city, town or village may be situated.