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):
Back view:
Update
Here is a zoomed out picture of the whole hole. The first picture is of the right side edge of the hole.
Best Answer
That looks suspiciously like somebody decided to cut a door through the wall and either gave up or decided it was too much work. The wall construction looks like veneer plaster, which would fit with the age of the house.
Whatever method you decide to use, you're going to have to get some more framing in there. You at very least would need to replace the stud in the middle that was cut off and either cut back far enough that you can hit a stud on either side of the hole (the option I'd go with), or put in enough blocking that you have solid edges. I'd just use sheet rock for the patch and shim it out so it is flush with the rest of the wall surface.