WW1 Era Paterson Walsh Aircraft Height Finder, for use at Sighting Stations

This is a heavy instrument which I believe was used in World War One (Patented in 1916 according to online information) but I don't know anything else about it.
There is no actual date on the box. It measures approx. 7 inches square and 4 inches deep.
I think it is one component of a kit. It is too heavy to post so, collection only please, from the Shoeburyness (Essex) area.