Okay, one of our club members helped me out a bit. I now understand that "U/C" was an old reference to the landing gear. I am putting in retracts so I will need to beef up the wing center section differently from the plans. His advise was to sheet the wing and then construct the bottom fuse pieces (belly pan) and glue to the wing.

Do any of you with completed wings please post a photo showing the belly pan on the wing?