You've probably posted it elsewhere, but is it possible to see the entire piece (flat shot, not at an angle.)
It looks like a large grooved bar weight. I've seen lots of them with tally marks along one or two edges. A lot of slate made it down to Kentucky in the Archaic times from Canada, Indiana, Ohio and New York.
There were two or three bar weights found at a bog site in Quebec, Canada with the atlatls still intact, and they were bound inline to atlatl lower on the handle. (Not up high and across the handle like a regular bannerstone.)
The tally marks might have represented something specific, might have helped in fastening the piece to the handle, or might have been purely decorational. Another theory on tally marks that I like is that little grooves and engravings would hold red ochre rubbed on the surface, and it made for a contrast.
The "mouth" does look like it is erroding out of a natural band, but the Archaic groups frequently used natural bands/swirls to show eyes, why not a mouth?
Very nice piece.