What a unique button you have there. It's not in Albert or Tice. I have been scratching my head thinking what it could be. The backmark "SCOVILLE MFG CO. WATERBURY" was used around 1860 and just before. Those early MSG buttons did not contain any stars on the backmark, and the "MFG" has an apostrophe between the "F" and the "G" in known examples. Your Missouri state seal on the front does not look like any 1850's and 1860's state buttons that I have seen. The details are not crisp, especially on the helmet and eagle, and that concerns me for a coat-sized button like this. I also cannot find any similar instances of Scoville using that unique font anywhere in the time period.
It's hard to tell from your photo, but is the button relatively flat? MSG buttons were cupcake topped, two-piece buttons.
While it *could* be a genuine button from an unidentified Scoville die variant form the 1850's, I believe that you have a replica. I have never seen a genuine button with details like yours. Were there ever reenactments at your battle site?
If you get a hankerin' to look at war and pre-war MSG buttons, Harry Ridgeway has a great photographic array on his website.