I've not had the drive shaft out of our hammer. There is a protrusion out the back of the rear yoke... I described it as a step-in, I think. My assumption is, it's "the donut"/the
bushing, but I don't know that as fact. The only thing coming to mind about the protrusion on a rear drive is it moves the drive pulley further from the frame. Yep, not very much but things like brake mechanism ...