Yep, those are burn marks.
This vintage Walter Johnson card seems to have been singed at some point on the bottom left corner. It's in pretty bad shape, but it's a Walter Johnson.
I remember getting this card about 10 years ago in Van Nuys, Calif. Some guy at a baseball card show was selling it. I'm not sure what I paid for it but if I were to guess I'd say about $100.
Today in NM condition it lists for $2,100. If it were only in NM condition.