Good question Paul !
Judging the photo, there would have been room behind the front seats, although
with the assumption, that the cockpit windows more or less were marking the edges
of the cabin, there hardly would have been much room for the rear seats. A possible
explanation, to my opinion, could be, that the S.20 was more or less designed as a
two seater, but could have carried, if “necessary, four people, although those on the
rear seats probably would have to had the stature of children. The same was true for
the Morane Saulnier MS.571, marketed as a "3 to 4 seat" aircraft. And taking into account
the load capacity of 450 kg, I can imagine, that it all performance data were based on
the use as a two-seater only.