My 100+ year old house has this odd piece of metal in an exterior wall. It looks like someone took a soup can bottom and inserted it. It's the exterior wall of the dining room, and there is no evidence of it on the interior, although I think the plaster walls may have been replaced in the 1940s. That part of the dining room is a walkway from the kitchen to the living room, and there is a fireplace on another wall of the room.

Was this just someone's cheap way of filling a knothole in the wood? I'm repairing and painting the wall, which is now a screened sunporch. Will post the photos as a reply.