This instrument is both a parallel rule and a rectangular protractor designed
for use with navigational charts. It is made of brass and has one wooden and one plastic handle. The edges are divided into degrees around the top and sides and marked clockwise every 10 degrees from 0 to 180 and also from 180 to 360. There are no scales along the bottom edge of the instrument. The mismatched handles seem to be a later addition.
In 1940 Kelvin & Wilfrid O. White advertised an all-metal Field’s Type Rule much like this one, instructing the user to "place one edge along the course and the index line on bottom edge of the meridian or vertical line on the chart and read the true course on the top edge." The catalogue also states that the instrument is "the quickest and most accurate rule available and is in general use on all seagoing vessels."
Ref: Kelvin & Wilfrid O. White Company, Catalogue of. . .
Navigational Instruments and Equipment (Boston, 1940), p. 35.