These are a list of three letter codes useful for specifying countries and counties (all within UK and Ireland).
The original list is at: http://www.lochinpublishing.org.uk/chapman_cc.htm
A modified list is at GENUKI: http://www.genuki.org.uk/big/Regions/Codes
A fuller list is Wikipedia: https://en.wikipedia.org/wiki/Chapman_code
Members may find the following useful
[Last updated: 21st February 2019]