It is number plates in Scotland. They have been SN and the last two digits of the year but it was realised this year it would be SN07 whichich could be read as SNOT. It's been changed to TN07 as a result.
Yes you're quite right, Edinburgh has SK to SO so it's a good job they didn't use the TO prefix when they had 55 plates or they could have had TO55 ERS...