Had to bump this thread as I found (what might be) my best corroded coin. I almost tossed it in my junk pocket in the field but noticed the round edge. Cleaned it up and it has the "19xx" (possibly 1911?) at the bottom! It's over 2mm thick and 20mm wide post-corrosion. Matching it against other coins it seems to best fit a nickel--maybe a Liberty Head? Would a Liberty Head corrode like this, so the copper is predominant? The "19" is actually in a lower layer than the surrounding copper, which is the "top" extant layer.
Whatever it is it's something I've never found, as have never found a Liberty Head, Indian, or Barber!
(Or any foreign coins like this. The "9" seems different than the US coins I compared it to, though that might just be due to corrosion, I guess)