The staff Working Timetables (WTTs) give details of the headcodes to be displayed, each volume having a summary in the introduction, with the actual headcode for each individual train given in the tables themselves.
Until 1967 / 1968 the WTTs were divided into four different areas:
Midland Lines (London Marylebone / St Pancras to Nottingham, Derby, Manchester and Liverpool);
Western Lines (London Euston to Crewe / Stoke and all lines west of that);
Central Lines (north of Liverpool / Manchester to Preston / Fleetwood, Hellifield and Skipton); and
North Western Lines (roughly Chester / Liverpool / Manchester and northwards to Carlisle).
Each of those areas were divided into lettered sections.
Below are some examples, with the cover, the map of the area covered, and the pages with the headcode summaries being shown.
These were from a 'combined volume' version containing all sections. The map shows the complete Midland Lines area, with a guide to each of the sections within.