Apparently there isn´t an STC for 206 club seating, considering it is a given configaration option by the factory. As a matter of fact, it is shown in the parts manual as an extra, just like wheel pants are, or cargo pack maybe. The noticeable difference is the seat-backs on the middle seats, they are way higher than normal, and seat belts are also also differently installed for those seats. Stowabe writting table is part of the option...

In case anyone is interested in 206 club seats, there are two sets on ebay. Enter "206 seats" in the search and scroll down to the bottom to the ebay store. Both sets are in beautiful 1970's colors. One set yellow-orange, the other alvacado green. Kinda spendy but hey, there they are.