Hmmm....Good question.
If you've got the 2010 catalogue to hand, have a close look at the photospread heading pages 82/3.
It shows the observation car, 12 wheeled Brake car No.94, and, nearest to the camera, a 12 wheeler with a combination of details not featured on any of the current releases.
The vestibule door has a full length oval glass, but without a cut-away roof cornice, similar to the brake. It's not a brake though, as the mainlight window spacings are different (narrower). Also, the panelling surrounding the oval toilet window is made up of 5 sections, instead of 4.
Possibly, this is a pre-production mock-up, or maybe there's a fourth model awaiting the go ahead ?