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Yes I saw that thanks.There's also one listed in the Maritime museum at Greenwich. I had been thinking of scaling off the picture but after a bit of puzzling the scale is quite simple. If I can get an 18 inch ruler marked in 10ths (not easy) I can 're label it 0 to 90 failing that I can print one onto perspex. I thought the device would simulate a standard plotting sheet using the altitude ruler and the longitude scale on the back but it's a bit more complicated than that and a bit of a puzzle as to how it should work. I'm assuming the white rules labelled "time scale" are used to transfer longitude from the scale on the back. These then slide up and down.  Curious as to what the numbers marked on the centre of the white rule signify. Does anyone have any experience of contacting museums for this sort of information. Google has drawn a blank so far.