Although the seat base is different for the Touring and Roadster/Std/Classic (RSC-which share the same seat fitting capability), the backrest that fits into the square hole and is fixed with 4 small bolts on the underside on the Touring's 'Long-haul' or the confusingly named RSC 'Touring' seat versions is the same although they may have different part numbers over time.
The backrests were sold separately to the seats and although the Roadster/Std/Classic backrest is no longer made or available from Triumph dealers, the seats are and are now discounted in Australia and the UK for AU$172 or near equivalent in the UK. The last NOS backrests in Australia were hunted down a few months ago and are now gone and sold for AU$320 without the seat itself. I know I got one of each still in packaging and have a used one as well also paired with a used seat.
I doubt anyone will sell them separately from the seat.