Thanks to my good friend, David W, for these pictures.
David calls this a semi-plateau or semi-Mazzeo, because the ring finger keys and left thumb key are plateau style, but with a hole in the middle. I would think that having the hole would be preferable, as it would prevent a dull, stuffy sound for middle C, low A, and open G if those pads were too close. On the other hand, a student needing a plateau style horn is likely going to have trouble covering the little hole in the middle. The instrument doesn't seem to have the most important part of the Mazzeo system, the Mazzeo register key mechanism, so semi-plateau would be the best description.
Normal clarients have all three rings connected, so when you push one down, all go down. However, this mechanism allows the last (plateau) key to move separately from the other two keys.