US Hawks Hang Gliding Association

I'm flying an old UP Comet. The down tubes connect to the keel with what was referred to as a Brock Box (I guess because Brock came up with it). There are 3 positions the apex of the control bar can be mounted in. Since the hang loop attaches in front of this connection, there is no interaction of the hang loop with the box. Can anyone tell me what the different positions will affect?

I dug out my old Comet manual. It doesn't even mention the three holes in the control bar box. I remember using the middle hole.

I made a Brock Box for my Moyes Midi out of stainless steel. (Drilling the holes were not all that easy)
The Moyes tow bar had us hanging from the downtubes and not the keel. So moving the position in the box moved the CG.
If you hang from the keel moving the Brock box position should not even make a noticeable difference.