This post mirrors my old imgur post on how to use original 645 focusing screens in the newer cameras.

Left the 645Z default focusing screen, right the 645 focusing screen with split screen and micro prism ring (UC-21). Focusing screens for 645N/D/Z are compatible (though the screen mounting frame on the 645Z is easier to handle than on the N), the original 645 screen has the same size and thickness as the later screens, but only one tab instead of the later two - and that one tab at a different location.

There is a microprism focusing screen for the 645N (AA-82) as well as a split screen focusing screen (AB-82), but both of them are almost impossible to find, and if they're available, ridiculously expensive - easily over EUR 200. The UC-21 on the other hand can be had for around 30 EUR, including shipping.

From this angle the tabs are easier to see.

To make the original screen 645N/D/Z compatible the notch needs to come off. I've found using an X-Acto knife to prepare the breaking point, and then just breaking it off with my multitool worked very well. It may be necessary to scrape a bit off the edge with the knife after breaking it off, though it doesn't need to be fully smooth to fit.

Important is to fit the screen with the right side up, otherwise focusing will not be accurate. In both old and new screens the notch is on the right side.

With the notch gone it's not easily possible to use the included tweezers to handle the screen - though even with the notch there they're not ideal. I recommend simply wearing cotton lab gloves, and using your fingers.

A crappy through the viewfinder image with the screen installed. The viewfinder seems to be slightly darker, but manual focusing with both microprism ring and the split screen works perfectly. Both the picture on the matte screen and the autofocus agree with those focusing aids, as does the final picture.