I've recently completely stripped and cleaned the rangefinder assembly in a Retina IIc. When I took out the little eyepiece lens I noted that it was fitted convex side inward, but when I checked in the official parts list available on Chris Sherlock's site, it says "Convex surface to rear of camera", so I guessed it must have been incorrectly installed at some time, and I replaced it flat side inwards.
Looking just now at Mike Eckman's site
I see that he says the lens should be convex side inward, as mine was originally. Which is it?