This pic is not as clear as I would have liked, but it does show enough to learn from.
The receptacle is a three-prong receptacle- quite an accomplishment for a K&T building. The open box above had a light switch in it, with the white wire used as a switch leg. The receptacle, and another farther down, were using the ground wire in the romex as their "neutral." Simply putting a receptacle tester in the outlet would yield an "OK" result.