The only possible evidence of your piece being worked is the in the last picture. If I had to try and make something out, I would say it resembles a face profile, with the nose (last picture) pointing up with pecked out eyes above and a grooved line for attaching to a stick below or made to look like a mouth.
Without examining the piece, this is the best I can do. There were many "nose" profile artifacts found throughout the continent.
I'm really stretching to make an id on your rock and only give my theory a small probability.
Try posting this on the Native American forum as fur trade is more specialized in historic metals.
Cheers,
Dave.