Not sure what the problem is but below, from the US Mamiya user forum, is a question and answer concerning which focus screens will work:
" Mdragoni - 06:20pm Sep 29, 2004 EST (3.)
Hi! I just bought a used manual focus 80mm macro to use with my 645AF...in the user manual, it says to first "mount the exclusive viewfinder screen #314474..." Is this really necessary? If so, why? And, if so, where the heck can I buy one? I can't find them anywhere online.
Danny Cirillo - 09:17am Sep 30, 2004 EST (4.)
Mamiya America Corporation
Michael: The microprism screen will make manual focusing easier, but it is not necessary. You may order it from any Mamiya dealer, it is catalog number 211-713. "
I do think there may be an issue regarding metering since with the "C" screen spot metering must be used. I meter mostly with a handheld and the histogram on the Phase One back so I haven't tested metering with the MF lenses and the two screens. I will though and report in a couple of weeks.