u/saturn-slurper

Complete beginner here! I found this in a box of leatherworking supplies at a thrift store- does anyone know what this is and what it's used for?

The top curved part is able to rotate relative to the bottom part, I'm not sure if that's supposed to be happening though. The bottom part looks like a blade of some kind. I think it is detachable, there's a set screw on the back of the tool that looks like it can be used to detach the blade.