Larry I'm guessing your home was built in the mid 60's to early 70's. You have some of the first "Dry Wall" products and like mentioned above where in 2' x 3' panels.
The older homes had lath (1 1/2" x 5/16" x random lengths of rough saw lumber) and plaster - you might find some lath still in your home.
Back when lath was installed it was soaked in water overnight and hammered up wet. The skim coated and the process of the plaster started. With these new backer units the smaller panel covered more ground more quickly and they went up as is and where "Dry" Hence the name Dry Wall for our current board.
Many of the men doing this work continued to use the same plaster and metal expanded corners for inside or outside sections. I've not seen a whole wall done with the expanded lath but the area you opened could show evidence that expanded lath was used in the corners of the dry wall panels.
Best way to drop that stuff fast is with a flat shovel. We wack the walls with the back side of the shovel and use the shovels edges to flip the metal of the wall.
Where a mask. Get some ventalation. Your in for a big mess!
Good Luck.
JW