Loose rangefinder insert on Rank Aldis (Mamiya 4b)

[yorksteve]

I have just acquired a Rank Aldis Ranger, a re-badged Mamiya 4b. All seemed to be in good order, apart from the rattling that was coming from the top, and the odd shapes appearing in the viewfinder. When I unscrewed the plate covering the viewfinder/rangefinder/meter windows, out fell the small piece of metal shown in the photo - the brightlines and rangefinder patch insert.

A simple question - where should it be?!

Should it go over the mirror, or over the part on the right of that cavity, which can be seen reflected in the mirror? Or immediately behind the cover which I took off?

Thanks in anticipation.