Hi Glyn,
All the S5, S6 and T6 frames are the same. The lug seen in your photo (to the rear of the downtube) is for the mounting of the footboards originally used on the touring T6 machines.
From memory, there are 3 attachment points for a sidecar - on the rear wheel lug, near the top of the seat tube (by the gearchange bellcrank), and through the front lower crosstube. If a 4th point is needed, it is usually clamped either to one or both of the front downtubes, just below the bottom of the petrol tank.
Regards,
Eddie.