Hi, seeking any opinions/idea on the historical roots of this mysterious "hole". :-)

I've been removing wallpaper from the walls of the room that's next to the kitchen. In removing one section, I saw an area that appears to be a cement "patch" for a hole in the wall.

Situational facts:

• House built in 1887/1888, Queen Anne in Massachusetts
  

• Chimneys of the house were for coal-burning (according to my chimney sweep guy, and evidence of patches in other rooms in walls that abut the chimney. However, this patch is not in front of the chimney in the wall.
  

• Walls are horsehair plaster, but no final 3rd coat. That is, I think the original builders stopped at the brown coat (2nd coat), because the owners intended to wallpaper the walls.
  

• The wallpaper layer I removed from this wall is not the original 1880's one. It's the most recent layer (because in another spot of the room behind a radiator, I found more layers of wallpaper).
  

• There's a door leading from this room to the kitchen room. A chimney runs in a cavity between the two rooms, and runs in the space behind the radiator that you see in the left of the picture. So there's a run of 19" between the room's wall with the patch that abuts the chimney and the kitchen wall that abuts the chimney. (I mean, so it feels like a little 19" 'hallway' when you pass through the doorway going into the kitchen).
  
  

The baseboard under the patch area is slightly different than the baseboard in the rest of the room. They did a good job of matching it, but I can see slight differences up close. The patch area is 29" high measured from the floor, and 18" wide for most of it, with that slight bump on the left edge near the top.

There's a very faint line that looks like a rectangular 'box' impression on the plaster wall that runs around the patched area. Maybe some decorative metal frame resting against the plaster wall, with a cavity behind grating?

I find the irregular shape of the left edge odd. I can see the horsehair plaster wall 'cut' kind of slanted:

More clues in the Basement:

The basement 'ceiling' is all open, and I can easily see the area that's directly under the patched wall area. Because the electric receptacle in the baseboard there is on a knob-and-tube circuit, I can identify the spot exactly and see the portion that's directly below the space behind the wall patch:

There is a large patched "hole" in the basement 'ceiling' at the spot:

This...