That was my thought too. I've seen the barrel flats grabbed in protected vise jaws and the crescent wrench(15") applied to the receiver top and bottom. Looked just like that.
Printable View
I learned a long time ago that assumptions and speculations are generally incorrect. So I have no idea how that wear of the finish occurred, but in my experience as a retired machinist, mechanic, and gunsmith, it would generally take multiple use of a wrench to cause such wear in my opinion. Which is mine and mine alone.