The last major change in Missouri County Lines was in 1861 with the creation of Worth County. [Of course, there was also the change in 1876 when the City of St. Louis seceded from St. Louis County.] That is not to say that there were no small changes in county boundaries to adjust, for example, to changes in river watercourses. In my publication "Missouri Post Offices, 1804-1981" I have a series of maps showing changes in county boundaries up to 1861. The book (out of print) should be available in the State Historical Societies.