This is a vintage branding iron called a "yakiin". These were/are used on wood, leather...food...anything you want to brand. Please see the last picture to see an example of how it can be used.
I think they would look fabulous if used in pottery as a stamp. Also...I think they could be used with ink but as it is metal, the image wouldn't be as smooth and clear as a rubber stamp -please keep that in mind if that is how you want to use it.