Harley-Davidson® FLHRS 2004-2007 Wide Smooth Touring One-Piece, No Studs. Mustang Part No. 75877. The Mustang new price is US$469.00
Info from the Mustang website: "Mustang’s one-piece ultra comfortable touring design provides the rider and passenger with the fullest, most comfortable seat possible. The seat nose is narrower than stock so that the rider can plant his feet on the ground. The baseplate is open in the middle to accept a Mustang or stock Harley® driver backrest kit. Road King seats will not work with an FLHT handrail."
I had this seat fitted to my 2007 Road King Custom. Fabulous long range touring seat and no grief from the pillion rider as the pilion seat is well padded.