Well... It's funny that you ask!
Corbin Doesn't make a seat for the 2005 Ridgeback. They make this seat for the 2004 Ridgeback with the 250 tire. With a little modification it fits perfect and the fit is actually better looking than on a 2004 bike. The seat sells for $650 from Corbin. I bought this one brand new, never used, and in the original box off Ebay for $150 shipped.
You can call Arlen Ness Big Dog Dealership and they do them.
Here is the deal, Ness owns the molds for the Corbin seats for a 2005 Ridgeback. So you can get this seat for an 05 but you have to buy it from Arlen Ness because Corbin doesn't have the rights to sell them. I don't know how much but I am sure it is at least $650 or more.
Here is the Actual Arlen Ness seat on a 2005 Ridgeback:
Here is Arlens info:
Arlen Ness Enterprises
6050 Dublin Blvd
Dublin, CA 94568
925-479-6300
Arlen Ness Motorcycles