Welcome aboard.. looks like a shaft abbrading stone and a pretty nice well used one for the type of stone. Here is one of mine that resembles yours. It's basically a sanding block for an arrow shaft..or other straight wooden shaft.
It has all of the characteristics of an abrader, in my opinion. Strange looking stone or lithic it is made of. Looks almost like a fossil that the natives used. However, I know nothing about fossils. I probably should not have replied here, but wanted to say good eye. Nice find!