I never grasped using more than heat and flexing to straighten a wooden shaft. Then roll it to find cause of wobble and heat it again.
I'm not saying a tool was never used. But the labor and time vs flexing by eye, which you'd still be doing anyways...
Your cut stone is neat!
I'd be using it to display a knife.
The cut was a little too deep on one end , but no problem...
As to it being a shaft straightener , the width looks like it would be for a quite small diameter shaft.
IF you can find examples of shaft diameters in the area you found the stone , more question could be asked.
Collecting material for shafts can involve gathering a bundle of them. Binding them helps them dry straighter. And judicious choices before collecting does too.
If a shaft is/was badly warped at one end it would be harder to straighten by hand alone. But with a bundle involved it could either be shortened on that end if long enough , or discarded from choices.