Butlers Rangers, you might be onto something there. Another possibility is that it was not defective but some experiment was started on that particular receiver that did not work out.
The area circled looks like it was milled out on purpose, maybe a sight was planned? It would be interesting to see it with the stock removed. If the "milled" flat area has a mounting hole and the whole area is hard, then it was done before heat treat at the arsenal and it is not some bubba's craziness.