There are actually three Mamiya 645 Pro prism finders - five if you include the prism finders that were available for the earlier Super bodies, but work fine on the Pro bodies.

There is the FK402 AE finder which has the cylindrical eyepiece. It is more of a penta-mirror construction than a true penta-prism - the corners show some distortion. It offers a meter and auto-exposure. Technically it was never offered separately, but instead only as part of the SVX kit.

Then there are the FP401 and FE401 prisms, with the FP401 being without a meter, while the FE401 offers a meter and auto-exposure. They are true prisms and relatively distortion free.

Finally there are the finders for the Super - the AE Prism finder N (similar to the FE401) and the non-metered Prism finder N (similar to the FP401).