I'm still guessing it was casting flash/ imperfection from the mold that got machined off instead of scrapping the casting. Looking at the marks still in the casting right below the "machined" part, looks like disc grinder marks in the mold to smooth off a flaw in the mold.
I used to make molds and repair them for die cast transmission housings and they literally run them till they break apart from the heat stress. Been flown to the Chrysler Kokomo casting plant to fix them a few times.
If that was converter damage hitting a way over there ...you would know it before it was taken apart!
Again not 100% sure but that's just my guess?
See the raised part/crack in the first picture beside the actual crack, that is from a crack in the mold surface and it comes out the opposite as a raised flaw in the casting.