It would appear to have nautical connections according to this:
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Quote:
Instrument : Range Finder Cotton Type MKII
Manufacturer : E.R. Watts & Son. London.
Country of origin : U.K.
Manufacturing year : Early 20th century
Like the stadimeter this instrument was used to measure the distance of an object from the observer.
A 4" long linear concave lens (in cross section front and rear of the glass are parallel) is fixed to the base. A prism is mounted on a carriage with an index mark that slides back and forth along the axis of the instrument. When at infinity the slanted side of the prism is parallel to the corresponding part of the concave lens and cancels out any light breaking. A distance to an object can be measured when the height (or width) of the object is known. The carriage is slid until both ends of the object coincide, similar to the use of a stadimeter or sextant.
Three reversible brass inserts calibrated on both sides in distance in yards, corresponding to various base heights, (20 and 25 feet, 30 and 35 feet and 40 and 50 feet), are fitted along the axis of the instrument. When the scale for the appropriate base height is selected and inserted, the carriage can be slid for coincidence and the distance read at the index mark on the sliding carriage.
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