That looks like plaster in the outlet to me. It's possible you have some unconventional thing going though--the baseboard is unusual!

You'll probably have lead paint and some GCs care about that. (They all should of course...)

California, right? The pictures remind me of houses I've seen in San Francisco. I'd guess the walls are stucco over concrete block. Anyway, why wonder? Be bold. Pick a spot on a wall you plan to tear out during your remodel and smack it with a hammer until you find out.

I would have it tested for asbestos first. I opened an account here at a testing lab and pay $6.00 a sample to have it tested. Why submit your lungs to harm?

Regarding asbestos, just fyi, for what it's worth, when I was pulling up 1953 linoleum (I started it, then hired a contractor to finish it), I called a local agency about asbestos to see what precautions should be taken, etc.

They told me that the area has to be larger than a certain square footage for the guidelines re asbestos to apply. Since my area was smaller than that, I didn't have to have asbestos specialists do anything special.

Still, I wore one of those paper breathing masks when I was working with it. The contractors didn't wear masks. They used a chain saw to cut out the entire flooring in large squares.

That was about 10 years ago. I'm happy to report that neither I nor my pets have any respiratory problems, so I guess it was okay.

My guess is a 'fiber' type board that is nailed perpendicular to the studs. It's usually 24" by 96". This was used in the 50's instead of lath.

It is then coated with 1/2" of rough plaster, and then a finish coat of plaster.

The backing board is quite resliant. I don't reccomend using a hammer on it. Find a place in a closet and make a small hole with a 1" spade bit, to verify what you have.

That's plaster. I agree with Geoffrey B...Probably rock lathe guessing from the age.

If you want to cut or drill to check, do so in a closet where it will be unnoticeable.

My house was built in '52 and it's plaster over rock lathe. Makes for a very strong soundproof wall.

Thanks, folks. My guess is rock lath, too. Doing a little reading about the kinds of cracks you get in rock lath matches, too. As does the age of the house.

I can do a test hole somewhere. I also have to take out a door frame on an internal door. Might be able to tell from there? And even easier: should't I be able to tell my just looking at the ceiling from attic?

Just demoed the ceramic tile in the kitchen and found the same fibre board underneath the tile mud. Doesn't look like rock lath, though--house was built in 1954. Any cracks in the plaster are not vertical.....and not many of them, either.

I have seen celotex used instead of rocklath; worked on a house with a 1930 remodel that used both. Celotex was passable for walls, a disaster when used on the ceilings; sagged terribly, as not enough rigidity on it's own. Looked like a canvas ceiling.
Casey