I dug this in a Union Civil War camp near Rolla, MO, that was primarily infantry but most had a mixed component. It was an early camp, probably 1861-1862. I found many typical general service buttons, but two of these thinner shield buttons.
According to the Albert button-book, it is a "generic" state or county Militia unit button, a "flat 2-piece" from the very early 1830s. It lacks stars around the eagle because it is a cuff-size button. See Albert button number GI-57.