Looks to be aftermarket... but I am not sure. Here is a pic of the bracket that the one side fits in. I would not have a problem taking these out for you and swapping rear carpets with you as well. As the carpet is cut out for the seat rails.
Here is a pic of the rails with the carpet installed
So, the rails mount on the floor on the far sides of the trunk area... The seat fits in the rails and there are pins that hold the seat in place. Pull the pins upward and the seat can be removed. When I first took it out it took me all of 30 seconds to do. you can see the pins in the 2nd pic down on of my original post (the pic of the back of the seat)