Sure looks like Bad arcing. I think I see some surface copper melt. I'd clean it up with a little 600 then 1000 grit paper. Blow the dust out afterward.
I wonder if this isn't insufficient brush pressure. Badly worn brushes put the springs at their lowest pressure. Light pressure can cause extreme heat, distempering the springs. I've seen springs turned into soft wire. I can't describe how to check this other than just replace them. <<<)))
Thank you for your reply. The brushes have been replaced already and there seems to be plenty of pressure from the springs. As for cleaning with the grit paper, as I mentioned, the commutator was rectified with a lathe already and supposedly the gaps between segments were cleaned. The pictures you see are after this was done and having ran the motor for about 3 hours with the new brushes. Still, would you say that I should clean with the grit paper? Thanks.