What in the World is it?

We're vacationing at our place at the beach this week, and several times I've picked up this small object that I found at a local flea market here last year.  The man I bought it from said that it had belonged to his grandfather, but he had no idea what it was used for.  To his knowledge it had always sat on a shelf - only being pulled out occasionally as a conversation starter.

I love the rich patina of the old wood and the pretty little porcelain knob on the handle.  The handle makes the little wood bars go back and forth within the block of wood.  There's nothing productive that takes place while rotating the handle, but it's one of those things you can do mindlessly.   I think it is an antique stress reducer, what do you think?  I seem to be less stressful already....but wait, maybe it's the vacation.