Or same price without the backrest at Procycles Horsby/St Peters Sydney
Triumph Touring Seat Dual Part A9708164, Triumph Rocket III & R-III Classic Accessories, from Procycles Australia
JCS Motorcycles, 287 Great Eastern Hwy, BURSWOOD, WA 6100 used to have them too for same but no longer listed.
A US forum member Speedy has fitted a hard wood timber post to the square hole in the seat and made a pretty good adjustable angle backrest to suit. Post #17 here:
Custom Solo Riders Backrest
Images of the factory back rest can be seen here:
I have one with the factory backrest but have also made my own for the Roadster two part Gel seats as the Classic one on the Roadster doesn't quite go back far enough for comfort for me with std pegs. I have since fitted footboards and extra pegs on the peg rail so will try it with those before selling.
Here is the link to the one I made with a std Roadster/ R3 pillion seat bracket.
Riders Back Rest