I now wonder if this is a fragment of Company B, 1st Arkansas Cavalry, State Troops, commanded by Captain Armstrong's Company of Johnson County
[Howerton, Bryan R.;"Re: Johnson County Volunteer Companies", Arkansas in the Civil War Message Board, Posted May 2011, Accessed 16 December 2011, http://history-sites.com/cgi-bin/bbs53x/arcwmb/webbbs_config.pl?noframes;read=23906>
]I also find some references to a couple of Texas units operating in Arkansas who were commanded by an Armstrong.