I assume you found that on the bottom of the finder for the 65mm? It is a parallax compensator. I think the idea is that the cold shoes for the original Press and the Super 23 were in slightly different places left-right, and if you used an old finder on the new camera, you could add that thing to center the finder. It is mentioned briefly in the Super 23 instructions, see for ex the bottom of
http://6x7.cz/mamiya-press-super-23/users-guide/instructions.html There may exist two kinds of 65mm finders, one with the shoe offset and one with it centered, not sure at the moment. Anyway, you probably don't need to worry about it unless shooting close-up. The up-down parallax is much more significant, and the finder has a manual adjustment for that.