Have you opened it (taken it out of the case) to see what's inside? I have one from 1860 in a case just like that. It had his age, ID, the date taken and a newspaper clipping about some of his war exploits.
My guess is a CS frock coat, based upon the fine weave of the fabric and the collar design, in addition to the coat size buttons. The spacing of the buttons throws me a bit, shame we can't see the skirt.