I hadn’t encountered this before. Maybe someone else can shed some light.

I was replacing an exterior door jamb a couple days ago. Without the old jamb in place, it was easy to see the cross-section of the exterior wall. I noticed that underneath what I always thought was stucco was a layer of some kind of drywall — where I would have expected to see plywood. Further, it doesn’t really look like the wall itself is thick enough for the stucco to have been applied to a regular wire mesh. The room is a master suite addition that was done in the late 70’s by the previous owner. We bought the house over three years ago, and I had noticed before that the stucco itself was a little crumbly. I had always chalked that up to some mixture problem with the material. (In fact, that stucco material was also used on a family room addition from the 50’s. It looks like the coating may have been added to the old addition when the new addition as finished. It’s crumbly, too. The house is in Northern California.)

Has anyone seen this kind of exterior finishing?