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
Original Indexes Research Notes
The Society has now added the Research Notes originally compiled by Original Indexes.
[Last updated: 18th January 2020]