23mm, Overcoat size Alabama Volunteer Corps (AVC) button, dug with blank depressed channel back.   Rare bird, indeed!  Growing up, my digging partner found one of these and treasures it among his collection. 
While a coat "AVC" is a very nice and desirable button, these "Overcoat-size Wings Down AVC" buttons are truly r-a-r-e.  This one appears to have been lost among the rocks and dented some or saw extensive usage.  It has gilt in all the right places and displays exceptionally well. It is priced at half the money of a perfect one.