In the rulebook under appendix G a list is published of all the National Sail Letters. The one currently published (2005-2008) has several flaws. Not surprising, as the world of nationality changes more often than in a cycle of four years. ISAF publishes on their web site a list called Member National Country Codes.
You should think with that, you might be able to update the one in the rule-book. Not so.
The reason I find this necessary for judging, has to do with an incident several years back when I went to an International Grade 1 event as a National Judge to get experience. I think this was in 2004, at the (then still called) SPA regatta. Together with two experienced IJ's I was on the water to do Immediate Penalties for breaking Rule 42 ( Appendix P).
We observed a laser (sorry Tillerman) perform his kinetics and decided that we would give him a yellow flag. The Laser had the letters CZE in his sail. I was the designated whistler and "flagger" for that race and - being not absolutely sure - asked the others, this is Czechoslovakia, correct? No answer.....
The rubber speeded up to get to a place where we could flag. Again I asked and the others nodded. I whistled and shouted to the sailor his nationality and sail number, while waiving the yellow flag. The sailor looked back and started his turns. While he was doing this, he was talking out loud and seem to swear. This is not altogether unusual. Most people don't like to be caught and punished.
After his gybe he shouted something towards us, but we didn't understand what. Getting closer he shouted again: "Czech Republic!" at the top of his lungs. Right..... we fell silent and realized he wasn't cross for getting caught, but because we identified him with the wrong nationality!
Let me make clear that this was my mistake and I apologized as soon as I was able. But the deed was done. Since then I make it a habit of going trough the boatpark, trying to find as many nationalities as I can, to learn them before I go on the water.
I've since then found a few who are not in the rule book and also not on the MNA list. For example, do you know what SCG stands for?
If you know others, please let me know. I don't want to get shouted at, again.