The Corbin backrest and the R3T seat with backrest are very different. I've had Corbins and now the Triumph. The latter bolts permanently fixes to the bottom of the seat with four bolt/nuts while the Corbin just drops in the hole in the seat. One can make it semi permanent by putting a bolt through the bottom before installing the seat. I never did that, preferring to be able to easily install and remove it.
There are used Corbin backrests on ebay ranging from about $50 to $150. I got mine for my 07 Classic on ebay in leather for $60 in like new condition.
That is a very nice seat at a good price. If I didn't already have the Triumph touring seat I'd be all over that like ugly on an ape.