Wish I had a receiver in hand to compare with but something is really different there. My first impression is it is not an accident, could be wrong here. Another photo with some light in those dark areas would be helpful. I am thinking I see half of a counter bore that should not be there.
Multiple cutters were mounted on the milling machines that did that work. To mill that step would have taken a lulu of a mistake if not done on purpose.
Assuming this receiver got the proper heat treatment, it should be safe to fire if everything else is normal. The sharp flat bottomed cut will weaken things but it is in a NON critical part of the receiver.
Hope a better photo will help figure out the puzzle.
Figured out what that half counter bore is, that is the pocket the trigger assembly fits in. The darkness in that area makes it hard to figure out.