Eric makes a valid point but I cannot say I see the likelihood of a chip-out. If it were a chip-out, I would expect it to follow the grain right through back of the panel. Also, the gradually increasing width of the gap, starting about under the back of the hammer, combined with the rounded terminus, looks a lot like an old mortise to me. The current width of the lower edge of the panel also appears quite consistent and I would not expect that to be the case if there was a chip-out. Just my opinion.