I suspect the difference in them is that the Std/tourer fender rails have a cut out in them for the mounting bracket on the backrest and the IS doesn't since the backrest is made into the trunk.
Why don't you just super Valk it and put the trunk on when riding two up? You can take the trunk off and put it back in a matter of minutes.
I've decided not to go that route for now as this will be a mostly solo bike. The sissy bar will be mainly for looks, and for the occasional 2 up scoot. May change my mind later on down the road. Gracias for the info.