Thanks for looking into this further and sorry for the delay.
The rocks.fbx didn't seem to have any missing faces, though rock.fbx did in one of the corners.
For some reason it happens with nearly all of the models -- 3 piece turret, alien craft 1, 2, 3, and 4, alien battlecruiser, all terrain pod drill, cannon barrel, etc.
The pivots are also way off center. Is that by design?
