Thanks Mike, but not what I'm asking. I am rather familiar with the ones you've given a link to,
The mirror is set at 22.5°, rather than the 45° that a typical diagonal is set at. Daniel's link is closer to the mark where the angle of reflection can be altered between 45° & 22.5°.
The Matsumoto EMS uses a pair of these to produce the ultimate 90° reflection. This arrangement of mirrors also produces a correctly oriented image too.
There is a pair of Matsumoto EMS's in the first picture. The third picture shows a DIY Matsumoto pair.