I recently purchased an old camera and attached to the case strap was a small leather case containing a small FFWB combiner's meter and instructions.
A little research revealed that it is a combined Range Finder and exposure meter made in Germany in the 1950's.
It fully works rangefinder wise, not sure about the exposure meter, although something appears to be happening. It appears they were made in black and this rarer Grey colour.
If anyone is interested in a full review of this little piece of history let me know..