Higbeam,
You've got a good eye there...that belt looks twisted on djradz"s picture.
Those check chains do help keep the front of the mower from dipping more than without, but there is still some variance in the front cut as the tractor goes up and down swales. It's the swivel link that djradz has attached to his top link that allows the rear of the mower to follow the contours and cut whatever the front missed. Such as when the tractor starts going on the downside of a swale and the front of the mower raises up.
I've got a lot of dips and bumps that are not gradual ones. The more I think about it, gauge wheels on the front of the mower and a floating top link seems like a good idea? What do you think?