I use the Nikon DG-2 2X Eyepiece Magnifier (mounted on my GSW690III) and the Nikon DR-3 Right-Angle Eyepiece Viewer (mounted on my GW670III) on my Fuji medium format rangefinders and my Nikon SLRs.
Wow! Super cool! I wish I had put this together when I had my GSW 680 - looks like it is way easier to hit critical focus in darker environments with the magnifier.