I have seen that set up with the grounds many times. Usually it is in houses built in the 1950's and early 60's. The grounds are terminated outside the box. Most common with that installation is all of the ground wires are crimped in a copper lug and then attached on the outside of the box. The most common attachment point is one of the screws that hold the box ears in place. If you are able to see past the edge of the top of the box into the wall you may see the lug. In some cases I was able to cut the wall open ever so slightly and insert a tiny screwdriver sideways into the screw slot to loosen the screw enough to free the lug. Then I cut the lug off and use needle nose pliers to separate the grounds. I can then pull the grounds back in the box by loosening the cable clamps. It is a slow process if you don't want to damage the wall. One box I found several months ago had the lug on the back of the box. I was lucky that I needed to cut a hole in the wall behind that box anyway.
I am surprised to hear of that ground arrangement on a house built in 1971. Maybe an old timer wired it or a DIYer who mimicked something he saw.