I just located Naturalization information for two of my Moniteau County relatives in an unexpected place. The Ancestry database "Passenger and Immigration Lists Index, 1500s-1900s" contains an index of Betty Williamson's transcriptions of Naturalizations from the Montieau County Circuit Court Books from the 1880s.
I pass on the information in hopes it might be of assistance to others.
The below is a quote detailing the source Bibliography on one of the records I located:
[WILLIAMSON, BETTY. "Moniteau County, MO, Naturalizations, Circuit Court Books 6-9." In Pioneer Times (Mid-Missouri Genealogical Society), vol. 15:4 (October 1991), pp. 199-204; vol. 16:1 (January 1992), pp. 6-14.]
Early indication is that the court records are at Missouri SOS Archives on microfilm. Can anyone confirm that?