On this parts fiche the Fr Prt.muff R is the most specified component.
This schematic depicts all components used in the Crf250r 2005 (5) Australia Exhaust Muffler (crf250r5) partslist.
A sample from the products for the Exhaust Muffler (crf250r5) include: Muffler Comp,fr, Washer,plain, 8mm, Fr Prt.muff R, Nut, Flange, 8mm.