I can't find the reference but I seem to remember Bill Maxwell has produced H screens, of what variety I don't know. It is doubtful if a split screen/microprism would allow more accurate manual focussing than using the focus arrows from the AF and/or the standard screen. I use a range of CF lenses, manual focus, on my H and focus using the standard screen, the arrows almost always confirm I am correct, and if not it usually a focus shift issue. Are you having problems with focus?