Travelcard
The original "zonal" system was mainly in Greater London. The zone areas used both letters and numbers for the outermost zones such that bus availability ignored the letter while BR and Underground availability depended also upon the letter(s) - e.g. a Travelcard or Capitalcard valid in zone 3a (but not 3b or 3c) was valid on buses in zones 3a, 3b and 3c but only in zone 3a when used on the railway services.