Can you show me the side view? I think that might be the AE Prism Finder N, which came out at the same time as the Mamiya 645 Super, and is compatible with both the 645 Super and the 645 Pro/Pro-Tl.
It might also be the version that coincided with the introduction of the 645 Pro - the AE Prism Finder FE401.
I don't know whether the eye cup was sold separately, or whether the same eyecup fit on all the prism finders, including the non-metering ones.
I had a similar eye cup that I was having problem locating for another prism so I called Mamiya in New York City [years ago] and they sold the last two they had to me.