Paralleloscope Holder Cab 13

[motto (RIP)]

I am not an artilleryman but as I understand the function of the paralleloscope or 7 man shaving mirror, it provides a reference that allows a true bearing to be maintained if the gun moves back under repeated firing as the spade digs in etc.
That's the 'parallel' part of the nomenclature, the tube can be kept parallel to the original bearing which cannot be done if simply referencing a fixed point. As Cletrac quotes, it is 'without parallax errors'.
Gun laying has always interested me and I would like to be corrected if I am wrong or fall short of a reasonable explanation.

David