Fits all American Tackle AW13, 3UL, U5, AL8 and AL10 reel seats. The fighting butt includes a threaded aluminum insert that attaches to the back end of the reel seat. The fighting butt screws into the insert, and includes an o-ring to provide a water tight seal. The fighting butt is removable, so you can choose when you want to use it. We recommend using a fighting butt cap when the fighting butt is removed (available separately - AFBC-T)