I can't quite tell...are the rims present on the blank side? If they are, it could be a struck through die cap. If they are not there, I would think it is post-mint damage.
A struck planchet sticks to the hammer die and effectively becomes the face of the die until it falls or wears off.
So there could be several coins struck before the blank planchet falls off ? Is there any value to this coin error, other than bid idiots on E-Bay ?
If the surface is as rough as it appears in the pic, I don't think it's a capped die. I'd suspect it's what Ripley said or from acid.
Ripley, What is the purpose of bead blasting ? Why would it be done ? D.T. The surface does appear to be rough, although pretty much even. If it was from acid, would not the whole coin be affected (both sides) ? Dave