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Martin

I can think of three possibilities:
1.         You are the first person ever to bother to read the instructions carefully and spot the mistake.  The UK ones come in plastic covers now with no instructions on them.

2.         The original writer might have been unfamiliar with the use of parallel for a line of latitude e.g. The 38^th Parallel.  Perhaps they just meant a line drawn parallel to the meridian.
3.         It will work as on a line of latitude, but you can’t pencil along the ‘left edge’.  You have to go round the corner onto the ‘top’ edge.

The only instruction I got was to stick a ‘Dayglow’ triangle near the N arrow, so I could find 0/360 easily.  If you've got an old thick one, be careful it doesn't get bent.  They snap easily. DaveP

I've got a WW II vintage Douglas Protractor.

There are instructions on the cardboard sleeve which start off: To lay off a course or bearing - Use the left side as a ruling edge and the graduation in italic figures on the meridian or parallel - rule the desired course or bearing. For example try 10°, 170°, 230°, 350°. The left edge can always be distinguished by the writing "Douglas Combined Plotter and Parallel Rule"

This sounds fine, and it's easy to lay off a course on the meridian.  But the instructions say "on the meridian or parallel" - why "parallel"? Thousands of these things must have been deployed, so it's not a typo.  How did they intend using a parallel? (Of course you could add or subtract 90° to use a parallel, but was that what they meant?)