I just purchased a home in the US built in 1965 and when we removed a piece of plywood we were told covered up the access to the pipes we actually found a huge (32 in x 48 in) hole in the wall.
I'm not 100% sure what we are looking at but the thickness of one side of the hole is a little more than 3/4in and the thickness on the other side of the hole is an inch. It look like there is a backing material that looks like drywall and then something over top of this. As far as I can learn from my google searches this might be plaster over drywall but I'm not 100% sure and I haven't been able to find any information on repairing this. Any suggestions.
Side view (sorry for the quality):
Update Here is a zoomed out picture of the whole hole. The first picture is of the right side edge of the hole.