I'm no historian or expert, but I suppose you are referring to the plunger rest. These were popular on those bows in the day since it made for more precise shooting with a particular arrow.
BTW, the C wood riser was actually more rare than the A or B, so finding them all over the place is not going to happen. With that said, they are also not as popular as the other two so prices are more reasonable when you find one.
That one looks like a fair deal...and comes with 3 sets of limbs too, right?