help IDing a hone

[Butzy]

Hello all,
Whilst down at my inlaw's lake house (which is pretty far off the grid) in south eastern Missouri, I came across this rock being sold as a "vintage knife sharpener"... I won't say what I payed for it, but it was a trivial amount.
I have my suspicions about what this may be, but I wanted to ask more experienced members what their thoughts were. good find, or "get what you pay for"?

it's about 8"x1.5" an as you can see, fairly thick. tapers down just slightly on one side.
Any info on what this might be would be great!

(p.s. i haven't put a razor to it yet, but i'll update when i have a chance to)