Unusual Mixing Bowl? Butter Maker?

I don't know the origin or history of this antique "mixing bowl", nor what's it's real function was. It was in our family home for decades. Is it for mixing something special? For butter? For something else? Why the drain spout? No markings on the bottom of the ceramic. I would love to know if anyone else has any information on this type of contraption. It now resides in the Gold Hill Museum in Colorado.