[QUOTE The biggest give away that it is a Slingshot would be the extra seat mount strap (thats exactly what it is) welded to the back of the frame to accomodate the Slingshot decal.[/QUOTE]
I agree. I had forgot about the rear nameplate under the seat. The slingshot has the same type of rear nameplate as your Duster does, just below & behind the seat, where the sissy bar mounts.
Whichever bike it is, I would recommend picking it up (if the price is right). It looks very nice and complete.