Yes, I my Euro Spec 280SL 1970, LHD, with kinder seat, was fitted with retractable belts when imported in late 80s. Looking at the way the retractor is fitted/fixed to the frame of Tosh's SL, is totally different to mine. My retractors are fitted to the frame in the floor behind the seats, (not to the side wall of the back shelf area). From the retractor, the belts travel around the seat bottom cushions up to a fixed end at the pillars frames, behind the shoulders. Of course, the belt is buckled to the center tunnel female buckles/latches. This method is done so that there is not a pass thru at the shoulders, and the use of the kinder seat is not interfered with. Only issue is that the seats do not travel all the way back to the frame wall when pushing seats back. But you are only loosing 1 1/2 inches.
Bob