This picture is taken from inside the pumpkin, looking out towards the wheel end of the long side.

It can look like a gouge/scratch in the resized pic, but it is a piece of wire(?) that seems to be fused to either side of the tube and broken in the middle.
There is no other marks, scores, gouges of any description that I can find. The only clue is the steel is a bit lumpy where the left piece of whatever it is joins the axle wall. The only thing I can come up with is when this diff was built, it's blown through with the MIG and the wire has fused to the opposite wall too. Then just left in there, and broken in half when the axle has been fitted. It doesn't look like there's been any repair work or anything on the housing. Any ideas?