It must have been an option, certainly in later models. I only have two rear seats fitted. I don't need more and prefer to have the extra space and less weight, being a caravanner. I was disappointed to find that, although from underneath the seats appear to have the sliding shaped base, the mechanism is not fitted. Reading the brochure before purchase (second-hand) had led me to believe that it was a standard fit.
P.S. Sorry about the blank page above -- don't know what happened there, another case of non-standard fit.