John Shriver , Apr 14, 2011; 09:54 p.m.
It's eyecup FG. Common to most of the ZX cameras, except for the ZX-M which is too cheap to have an eyecup.
Easiest way is to probably buy one the cheap "double digit" ZX cameras with the "crippled" lens mount that still has it. Or one with a shot mirror motor gear.
My own MZ-3 came without one, haven't addressed the problem yet. I did buy a ZX-M for the split-image screen and the non-Quartz-Date back.