I just had one installed by a plumber. After he left, I lifted the tank to look inside and noticed the back right hand top of the tank had a chunk missing from it. When I called to complain that I wanted the defective toilet replaced by one that didn't have a piece missing, I was told by the plumber that this is how all the Toto tanks were designed. Judging from some of the pictures of open tank tops on this site - that doesn't seem to be the case. Do any of you who own Toto toliets have this "cut-away" feature to allow air to flow in the tank (as the plumber put it). Or am I being duped?