Dear Bingley: I didn't get the point of your post until just now. In WWI, it would have been suicidal to put your head up above trench to use binoculars. So a 45degree mirror
overhead allowed safe use of binoculars. From Internet:

[page 185] Observation from behind cover or protected from enemy
sight has always been a necessity in the military. The oldest device
for this purpose is the Polemoscope, a stand with two parallel pairs of
mirrors at 45 degrees to the tube. 'Hevelius maintains that he had
invented this Polemoscope in 1637' (Pisko 1869, where on page 51 is a
picture of such an instrument. See bibliography.)