Since I was waiting for engine parts, I decided to drive up to the farm and pick up the seats from a '91 560SEL that I bought 4 or 5 years ago. I got them and then took out the old broken down ones that were in the car. Here is a comparison of the front passenger seats.
This is the area of the car with the seat removed. You can see the four bolt locations. I've cleaned this area up a little. I found some change and a fuel receipt from April 2002. Maybe the last time this car was running.
You need to be able to move the seat up to get at the rear bolts for removal. The same with the fron ones. It helps to move the seal back. I used a 10.. socket with a universal joint. While the rear bolts are angled, you still can't get a straight shot at them.