I could post a picture, but not at home right now . . .
My house was built in 1964 (Arlington, Va) -- it's brick all around and the walls are "plasterboard". I've lived in it for about 2 years but haven't done interior work and am puzzled by the plasterboard. The boards appear to be like drywall sheets mounted to studs (most likely nailed in), and the cheap electronic stud finders are of no use, and even rare earth magnets are no use (like the "thud" finder. The magnet can find baseboard nails, but often nails miss studs.
Well, my questyion is how to patch cut-outs (I'm covering up large cavities that were cut for alarm keypads mounted to recessed metal frames; new keypads are smaller and can be mounted to standard outlet boxes). Some cavities need to be patched on the sides.
The plaster boards appear to be 7/8" thick -- boards have 3 layers -- a finished layer about 1/32-1/16" thick, a main plaster layer (gray color) that is about 2/3 pct thick, and a 1/3 back layer (eggshell white). I didn't detect if paper sandwiched the layers (paper backing of course)
I thought the thickest repair drywall is 3/4", so just looking at this and without burning gas to HD or another supply store, I am stumped on how to fill large cuts.