The Zonal Fare System was first instigated by the Greater London Council (GLC) under the leadership of Ken Livingstone back in the 1980s.
Back then there were far fewer zones than today... three. Oh well, not really - that's cheating - 'cos there was 3a, 3b and 3c. So that's five zones.
Anyway, semantics aside, rather than overlay the zonal boundaries onto the main tube map at the time, LU cleverly displayed this 'zonal map' at the back of their route diagram leaflet during the mid-1980s.
Note that only terminal stations and those at zonal boundaries have their names shown; the rest simply have an unlabelled tick-mark. It could be argued that this was a much simpler way of conveying this information.