Looks like he replaced the pillion seat with one of the accessory infills (there are 2) from Triumph - you can check it out on the Triumph site.Where did you get the seat with no pillion?
If yours is the 3R, this is the infill you need to replace the pillion.
PILLION INFILL - SPORTS
PART NUMBER: A9700879The pillion infill matches and contours perfectly to the Sports Rider Seat, changing your Rocket into a single seater motorcycle whilst concealing pillion seat fixings and revealing more of the rear mudguard. Features a stylish Triumph badge on the rear.
$156.000