I am inclined to lean toward SB's idea somewhat, but with a little different angle. Groove side is not belly, it' the back.
I think it is a groove for a cable backing and that is why there are 2 sets of knock grooves and the notch in the top as well. bow string in the outer set and cable backing over the top and bottom notches to wrap the opposite direction as the bow string in the outer set of nocks. I have been thinking on something like this for a while.
I think it would have to have the cable applied first and then strung up for shooting.