This is a mid-war Rangefinder Infantry, No. 2. (MK III) production No. 3737, which of course shows signs of use, as these were in service on nearly every British Royal Warship until the advent of radar.
This was one of the most optically complex instruments in World War I. They could have even been taken into the fields, but trench warfare mitigated the lack of a need for distance sight. The distance of the enemy was usually rather fixed. This is not to say that it did not see use in Flanders Fields. The Barr & Stroud rangefinder in the collection of the University of Leeds is said to have been "used by the infantry in WW1, and presented to the University in 1923."
Engineering professor Archibald Barr (Glasgow) and Physics professor William Stroud (Leeds) founded an optics manufacturing company in Glasgow, Scotland, in 1895, and the decades of wars made it so lucrative that the two doctors were full-time businessmen at the outbreak of World War I.
Anniesland, Glasgow, Scotland, 1916.
The rangefinder has a 4 inch diameter scope, but the apparatus measures 35 1/2 x 7. Weighs around 20 lbs.