Mamiya 645 PRO TL - Manual (Page 44)

Things to Watch
* When using the AE Prism Finder FE401
When using this finder it is imperative that the Film
Speed Dial on the Roll Film Holder is properly set for
the IS0 number of the loaded film, as it interfaces with
this finder. If not done, wrong exposures may result.
* When the mirror is locked in the up position.
The Focal Plane Shutter Curtain may be damaged if
the camera faces strong light sources, especially the
sun. Return mirror to normal position or use lens cap
to prevent such damage.