I think that the Mamiya option is history.
A problem seems to be that the RB67 screen is designed to be used with its
cover glass. In the Rolleiflex only the screen is used, not the cover glass.
The screen itself is not flat but is bowed upward. That will cause a focusing
error. With the cover glass mounted on top of it the focusing screen is held
flat. Some people say they have used this procedure successfully but I feel it
is not advisable. I have no personal experience. It's just what I have read.
The original poster of the Mamiya option has deleted his instruction page some
years ago. I still have the text and pictures.
From my MacBook.
Op 18 jul. 2015, om 14:06 heeft John Wild <JWild@xxxxxxxxxxxxxxxxxxxxxx> het
I have read that people carefully cut down a Mamiya RB67 screen to fit and
the results are a great improvement in brightness and snapping into focus. I
can't comment on viewfinder re-focus requirement.
David Runyard has done this for his SL66 - see Club Roillei User Issue 8,