So for a "shelf" car there should be 2 rails and for a "seat" car, just one?
No. There is only one rail, on all W113 Pagoda cars except the 250/280SL California type (rear bench seat, no soft top).
The rail is situated slightly to the left of the middle line of the car. As per Steven's picture.
This rail, with its two D shaped fittings, serves both purposes. To support shelves, or Kinder transverse seat pad (when that extra option is fitted).
I borrow a picture here, for further illustration