So I was reading a post about getting dust out of your viewfinder (mine got some small hairs) and the op mentioned his mistake of scratching the filter and I laughed because I knew how careful you have to be when handling the focus screens....or so I thought. Right as I was handling my focus screen after blowing out the camera I slipped and my nail scratched it.
So then I get the bright idea that maybe I didn't scratch it but just got some nail residue on it and proceeded to wipe it off with a lens cloth and it smears the whole thing.....
So now I have my 5D II and an unusable focus screen with no spares and I remember...I still have my old 5D focus screens. I get them out and alas they don't fit. I take a closer look as to why and it appears the only difference between the 5D and 5D II screens is two small tabs on the left and right side. So I cut the tabs off carefully with scissors and I now have a 5D focus screen in my 5D II to use until I get another one. It looks like they designed them purposely to be incompatible even though they are almost identical.
Just thought I would share my experience. Watch those finger nails.