In the former butler's pantry (now part of the new kitchen) there was a hole in the wall We were told that the the hole was made by the former owner for a window air conditioner. The old casement windows have openings that are too small for an A/C unit to fit, so they tore out a 2'x2' hole in the the 14" thick concrete wall. When we bought the house the hole was covered with plywood on the inside and outside.
We removed the plywood to do a permanent repair of the hole. When we removed the plywood we found this: