There are four drake bowls and one tank for the 1.6 gallon.
The only difference between the elongated and round bowl is the 1.5" at the end of the bowl.
You can see the same thing by looking at the side.
Same bowl, Same trapway, but one bowl extends just a little longer where it meets the end of the seat.
Have you run a auger down the trapway to make sure the kids aren't dropping toys in the bowl?