rduraoc: The image shows the rangefinder in the Leica M6. It will look different in a Fed 4. For instance the framelines may be painted on the glass and not projected onto it etc.
The Leica M5/M6/M7/MP uses TTL metering. The meter works like this: Light enters through the lens and is reflected by a filled white circle on the shutter curtain. The reflection is read by a sensor pointing towards the circle (see http://www.leica-camera.com/imperia/.../4_290x292.gif ). It is quite possible the Fed uses a non-TTL meter and that it is placed behind the middle window.