I received my replacement focusing screen a few days ago and today dropped in to the friendly Nikon Repair Facility at FotoCare (Oslo) to get their assistance in installing it.
The screen was a K3 (split-image centre with a surrounding "doughnut" comprised of microprisms). It is sourced from a factory made screen for the FM3A. My Nikon tech said the screen itself had been expertly modified and thus fitted the camera nicely. However, the plastic shims packaged with the screen were too flimsy and fragile to stay really put for a work-horse camera. Thus, after seeing the shims actually shifted to the extent their edges became visible in the finder when using the camera, we found jointly out that putting in genuine metal factory shims would be the only viable approach.
The camera now focused even the brightest f/1.2 lenses perfectly and neither the split image nor microprisms blacken out with my 50-300/4.5 ED Zoom-Nikkor. I found the screen to be perceptibly much brighter than I had expected.
I got it from
www.focusingscreen.com. They have a good selection of replacement screens and ship world wide. Quite acceptable prices too once you realise they quote in Taiwan dollars.