Antique brass rat's head? Trying to figure out what this is...

I haven't any idea what this is. It looks, to me, like a rat's head. It is rather heavy and comes unscrewed but there is little room inside. It came in this fitted case. There is a tag on it that is metallic that reads "Katherine's collection" but I do not know if that is the original tag. The case and the item both look very old. Can someone help me identify this item and its purpose? Thanks.