EC1: why this London postcode has the number 1

EC1 is one of the six numbers of central London in the Post Office's list of March 1917: EC1 to EC4 for the City, WC1 and WC2 to the west of it. The General Post Office signed its notice of 1 March 1917 from “St. Martin's-le-Grand, E.C. 1”. For these six the list prints streets and no office names, so this page cannot say which office each number stood for.