I'd want to go at the line with a thumbnail or a piece of 4/0 steel wool.
I feel it's on the surface, and could/would wipe off.
It could be from a stray cleaning patch fiber caught in a liberally lubricated action when it was closed. Notice the red goo above the lower latch?
As for the circular scar, it looks like a screwdriver turned by hand in a milling machine or drillpress spindle broke because the dumb*** didn't remove the retaining screw before trying to get the bushing out. It looks to me like a gall mark from a broken tool.
Isn't gas cutting smoother than that?