[darrenf]

Zone 5 was split into two back in 1991.

Zones beyond 6 have existed since at least the late 90s. My vague recollection is that A-C basically meant "the top end of the Metropolitan line" - certainly how it seemed for me as an SW Londoner. Then at some point the letters were changed to numbers and more stations were brought inside over in Essex, Kent, etc. The Wikipedia page for stations in zones 7-9 has a changelog of sorts: https://en.wikipedia.org/wiki/List_of_stations_in_London_far...