A ring mandrel, also known as a ring sizer or ring gauge. It is a tool used for measuring the size of rings. This particular mandrel appears to be made of metal, likely aluminum, and features grooved markings along its length indicating ring sizes. These markings allow jewelers and individuals to accurately determine the size of a ring or a person's finger. Ring mandrels are essential tools in jewelry making and repair, ensuring a proper fit for rings.They can also be used to reshape or resize rings.

I would use it personally to size the opening of my bottles to be able to by the right corks or caps. :)