Is that slide rule that's in the picture, or is it a substitute for a set of parallel rules? . . .
Tags in this image: slide rule, dictionary, grid paper, compass, map, pencil, pens, ruler
The ruler looks like it has a roller built in so that it can hold a bearing from the compass rose to wherever you need to draw a course (vice versa); so that would be a substitute for parallels.
I've got all that here, except that the parallels have been replaced by an Air Navigation Plotter which is a
protractor designed for navigation.