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I now have copies of the 1976 and 2008 versions of the London Nameplate Manufacturing Company Ltd’s instructions for the Douglas Protractor sent to me by Gloria from that company (photos attached).  There seems to have been a gradual simplification and tidying up of the language over the years.  By 1976, all reference to lining up on a meridian or parallel had been changed to lining up on a meridian of longitude.  The instructions for plotting positions from horizontal angles are still rather complicated.  The reason for turning the protractor over must be to reduce parallax errors, because a good protractor will be about 1.5mm thick.  You don’t need to do this if you use the protractor to mark the bearings onto tracing paper first and just manoeuvre the tracing paper.  To avoid giving photo credits, I’ve not included a copy of the earlier instructions which include reference to parallels of latitude, but they can be viewed here https://progress-is-fine.blogspot.co.uk/2014/08/douglas-combined-protractor-parallel.html#!/2014/08/douglas-combined-protractor-parallel.html .  DaveP

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