The difference is in the back of the seat. S1's have a properly trimmed back with a roll of crash cushion at the top. S2's have a fibreglass panel covered in leathercloth secured with a single large chrome screw near the top centre. there are also minor differences to the side of the backrest low down to accomodate inertia reel belts.
The S1 seat carried over to the early S2's changing to the seat I described as the S2 seat around the 72/73 model year change. I've always thought the S1 seat was a much classier production. But many cars have had the interior changed and S2 seats fitted.
So, yes Al, your seats are original!
The box pleat / flat pleat discusion covers both seat types. Box pleat vinyl (which was offered on early 3500S to keep the price down) and leather were confined to 3500S (with both S1 and S2 style backs), flat pleat, as Al's, were for 2000's and 3500's. Later on things get a little muddier and certainly many late 2200TC's have box pleat leather. All cloth seats are broadly box pleat in style, except for a few S1's which were offered with a different type of cloth. There are vanishingly few of those around though.
Chris
