350 SL (R107) rear seat belts

Hi list,

I'm new to the list and try to write in my best english, so forgive me in advance...

The topic has been already discussed but I haven't found the detailed infos: can anyone explain how do you attach a two-points lap belt on the rear seat of an W107 (I've no rear seat, just a wood bench...)? I've found 4 anchor points: one screw on each side (after removing the side plastic moldings) that seems to be the top shock absorber mounts but will "fit" for the belt, and in the middle of each "seat", one hole and one metal plate.
My goal is to put a baby seat (for short country side trip, obviously...). Should I go from one hole to the other one, making a centered seat? Or should I go from one of the side screw to one of the hole (which one then?)? What about the plates? I can't make the belts goes thru, they are too thick...Last question: in the side plastic moldings, I've found pre-cut holes that indicates that the belt should go through...How come? Belts should be in direct tension, if I don't get wrong...

Thanks for any tips,

Thomas
1979 350 SL
Paris, France