"A" "U" and "S" in black font. Six required per sail (three letters per side). Country letters are required by ILCA class rules when competing at Regional and World Championship events. This includes ILCA Oceania and European Championship events.
230mm Country Letters are used on the ILCA 4 sail and 300mm letters are used in the ILCA 6 and ILCA 7 class.
Instructions for the applying sail numbers and letters can be found here, alongside instructional videos.