I'm not very familiar with that type of construction, but from the picture my guess would be its a layer of masonry under the stucco coating. In older houses they used to install a cement layer on vertical surfaces (not cement board like they now use) when they were going to tile them, and maybe they used a similar underlayment for stucco. The walls of my 1940 brick house are about 13" thick from the exterior brick surface to the inside of the plaster, so it takes a while to drill through all the layers for bath and kitchen vents.

Modern stucco is just applied on top of foam board and wire mesh, so its much different.

My house also has a fairly thick layer of cement under the tile floors, and they had to build a recessed tray in the flooring to provide space for the cement layer. The cement base was apparently applied before the plaster on the walls, because I found that excess cement had gone over into the bottom of the stud bays in those rooms.

Bruce

This is typical rectangle hole for range hood vent, 10"x3.5", I just drill a lock size hole on the wood. it is about 70 inches above the floor and in the middle of the wall to the ceiling. usually there would be vertical studs, however this wood is at lease more than 10 inch in horizontal and more than 3.5 inch in vertical way, and it is 2.5 inch thick. I am expecting an insulation layer there, this is a house round 1930 . It could be a structural part of the house, however what could this kind of wall structure be?

We have something that looks similar in our exterior walls, but it's hard to tell from the photos if it's really the same thing. Our walls contain "wood nogging", which is similar to adobe. I think it was used in our area (interior western US) as insulation - and it provides a lot of thermal mass but has a pretty low R-value. Whenever possible we removed it and replaced it with high R-value insulation.

I wonder if the house started out as a barn?

Do you suppose it could be plank frame construction?

Maybe you need a professional.

Rock Lath? It was big back in the early 1900's when someone did not want to nail up all of the lath strips.

Is that picture taken from inside the home or outside? And is that bark I see on that vertical piece of wood on the left? Please post an additional photo or two—from further away, please.