Hi aoluain:
The Super was designed to take the following finders: Waist Level Finder N, Prism Finder N and AE Prism Finder N.
As indicated in the manual for the camera, the Pro was designed to take the following finders: Waist Level Finder N, Prism Finder FP401 and AE Prism Finder FE401, but viewfinders for the M645 Super can be used with it.
If I recall correctly, there is a problem if you try to use an AE Prism Finder FE401 on the Super.
There is also another finder that one finds from time to time on the Pro - it is the SV AE Reflex Finder or SVX non-metering Reflex Finder. These finders have the variable diopter, and again, If I recall correctly, there is a problem if you try to use an SV AE Reflex Finder on the Super.
I have used both the metering and non-metering N prism finders on both the Pro and the Super, without problem.
Matt