Hey there,

Sorry this is embarrasingly late. The brass plate is really a grounding plate. I typically make a ground bus out of wire and solder it across the back of the pots, then solder all the ground points to that. The end then goes to ground at the three terminal strip. You can also do this with a ground wire. The newer kits come with a strip of ground wire (squarish strip) that you can attach across the pots.