Looking for the name of an item that I will try to describe in the details.

It is a small metal piece. When flat, it is round on the bottom with a tab at the top. When I was a child & I went to somewhere like a museum, after I paid I would get one & you would either put the tab through a button hole & bend it or bend it around a collar lapel. It would have the name of the place on the round part & the bent tab would be hidden & would hold it in place. Not sure how else to describe it. Anyone have any idea of what this would be called? I don’t even know what words to try to do an online search for it. Thanks very much for your time!