Reeves, that style of flat bottom isn't terribly common on ancient pottery from the Eastern US. It shows up on some styles (cups/mugs/cylinders) more than others, but it isn't nearly as common here as it is in Central American pottery where it would be more common than not. It became a more common feature on later made for trade pieces that were intended to sit on a table or shelf (a hard flat surface), like much of the Catawba pottery, in the 1800s and later.
I really don't know what it is other than to say I don't think it's normal Mississippian pottery from the eastern part of the US.