If it does turn out to be a back-silvered mirror, and you feel like replacing it with a front-silvered one, look out for a cheap plastic Polaroid, of the type that has the film-pack underneath. They cost next to nothing and have a mirror big enough to make several rangefinder mirrors. Use the glass-cutter on the glass side, not the silvered side.

If you haven't already seen it, there's some information on the camera on Daniel Mitchell's site.