This arose in Another Place, where only a partial answer was available.    I hope you can tell me.   Wiring diagrams in the Haynes Spitfire manual contain variations on a simple symbol, but no translation.   The symbol may appear as a diagonally crossed circle, the same with alternate quarters black and white, or a circle half black and white, with the dividing diameter either vertical or horizontal.  See below.    A  diagonal crossed circle is an alternative symbol for a light bulb, but