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Geoffrey Kolbe
Thank you for answering David Harrison. When I mentioned unknown magnetic variation, I wasn’t thinking specifically of motor vehicles where magnetic deviation is a separate problem and another reason for using a Sun compass.  I was thinking of those parts of the Earth where, even in the 1930s, accurate variation was unknown.  Possibly not Libya, but certainly in Northern Canada and the Greenland Interior.   The latter two are also areas of low horizontal component of earth magnetism, which would also cause sluggishness in a magnetic compass. 

In so far as lugging a complete copy of Davies around, there wouldn’t be much change in latitude or longitude if you were on foot , so you need only take the relevant daily pages for GHA and declination Sun plus the relevant latitude pages for azimuth with you tucked up in your sleeping bag.  Each evening you could complete a pencilled table on a prepared postcard of celluloid or similar of the azimuth plate value for each hour to be set against the lubber line to ensure that when the Sun’s shadow was a thin line on 360, the lubber line pointed in the direction of your track required. The more I look at that gnomon, the more I think higher latitudes.  Knowing the angle between the azimuth plate and a line from the inner ring of the azimuth calibration to the tip of the gnomon should allow you to calculate the minimum latitude at which the device could be used at noon on the summer soltice.

Rough sketches on the back of an envelope show the formulae on the handle appear work north of the Tropic of Cancer.  I’ll try and find time to look at all possibilities and maybe come up with a diagram and an example of a table.  DaveP