This X-812 is needed to mount any Canon 5D-series (incl. Mk3) SLR to the X2-PRO system.
The 5D should be mounted with the Canon Battery Grip in place.
The X-812 places the CCD exactly in the optical center of the lens, in horizontal as well as vertical position.
The X-812 will also host Canon 7D and 20/30/40D and 50D all with Battery Grip.
Note that the tripod mount of the Mk1&2 versions had the 1/4" thread on an asymmetrical place, while the Mk3 is mounted in the center of the block.
The later versions of the X- 812 feature a slot that makes positioning of both possible with the lens in the center of the camera.