It's a 1.3x magnifier for Bessa R2x/R3x, Epson R-D1, Zeiss Ikon and of course, Nikon FM/FE series (that's why it's called "1.3N"). Just take off the camera's eyepiece and screw it on.
It's available at Megaperls and someone on the R-D1 forum has it. I'm not going to get the Epson but may trade my M6TTL for the ZI, hence the question: is it a good match for the ZI's 90mm framelines?