12-11-2012 02:57 AM - edited 12-11-2012 09:53 AM
I see from the spec that the camera uses a new Eg-A II focussing screen as standard though the older Eg-A screen will fit you are likely to get exposure errors. The EOS 6D instruction manual lists the older Eg-S (super precision matte) and Eg-D (grid) fit too (page 304 / 354). There's a custom function (C.Fn III-2) in the camera to optimise the metering for the different screens.
The Eg-S and Eg-D are the same focussing screens from the EOS 5D Mark II and also the original EOS 5D.