This was in a basement of a house built in 1930 on the underside of the sub flooring. It felt like a metal grid with hard cardboard-ish material in between. It was on the underside of multiple areas of the basement subfloor. Any help would be greatly appreciated! Thanks!
Looks to me to be a wire mesh with a mortar bed for tile work, that was the way they did it back in the day.
What Neil said.