That is the Gel seat. Notorious for burnt bums as the gel gets VERY hot in the sun.
ROADSTER GEL SEAT - DUAL A9701140
Why you not want the much better and more comfortable.
DUAL SEAT KIT - CLASSIC A9708059
or
CLASSIC TOURING SEAT - DUAL A9708164
For use with AdjustableRider Backrest A9708160
Shown with ADJUSTABLE RIDER BACKREST A9708160
This comes with the seat and riders backrest sold separately. The seat has a plug that fills the hole when backrest not fitted. The backrest is no longer available from Triumph but the seat is still. A few of us have made Rider's backrests to fit.