I think something pressed up against it and caused that effect. If it was going to be designed, it would either be raised, or indented as in etching. But the good news is you seldom find those that other goodies don't come out. And I even got to hunt today for the first time in about four years. Found an eagle button, but the no good, dirty, stinking hunting partner found a CS tongue. We are making him walk home.
I have a button like yours and it has a bunch of indentation dots in a cluster,vanzutphen