- How can I turn on 4096x1440 @ 25p (1:1 crop) on my 5dMk3?

While I've sucessfully recorded in 3.5K 1:1 centered x5, I can't figure out how to turn on 4096x1440 @ 25p (1:1 crop).
No combination of Crop mode and RAW video in the ML menu seemed to achieve that resolution.
Did I forget to make some settings in a certain module?