I have a late IIIc (small c) and the rangefinder is very dirty, I am trying to remove the top cover but I cant get it off. The meter (late single range) seems to be stopping me, how do you remove the meter dial?
I don't know whether it was the later lightmeter version, but on the only IIIc I ever opened the lightmeter dial came off together with the top. Don't turn it with the top off in order not to loose coupling with the match-needle mechanism.
Usually a Retina rangefinder does not improve much with cleaning. You also will have to clean the front lens of the viewfinder which consists of several pieces of glass, frames etc. Leave a note to make sure to insert everything in proper order.