Missell probably belonged to one of the companies of Dale County 1st Class Militia. The state legislature abolished all earlier militia organizations in August 1863. New companies organized under the law in 1864-65 were either 1st or 2nd Class Militia. No other state organization was allowed.
I have found correspondence and papers concering militia exemptions to be quite helpful. These are filed by county at the state archives and have not been indexed or made available via internet.