Yes, it's the part that the laterals screw into, that sits on the bottom...when the laterals are seated in place, there shouldn't be any space between the laterals and the part they're connected to. (Sorry that may not be the best description, but I didn't think to take pics while we had it apart.) Mine were seated in as far as they would go, but it appeared as if they were not screwed all the way in, creating about 1/2" of space between the end of the lateral and the hub, on three of the laterals. Further, The hub has an opening underneath it where the filtered water goes back up into the center into a pipe, and that pipe connects to an L-shaped piece of pipe that goes back out to the multiport valve. It is the connectioin between this L-shaped pipe and the hub pipe that the sand got in between and literally sanded down the inside pipe at the joint, creating space between the two pipes for more sand to enter the return stream.

Does that help?

Janet