A Wikipedia user in Uruguay, Mauri Manya, says that the Distinguishing Sign for Uruguay has changed to UY "al adoptar el sistema unificado en todo el pas" (on the adoption of a unified system throughout the country).
IANA has chosen to use ISO 3166 as a basis for the cc TLDs (see their list ), and sticks to it very closely.
It does, however, use as well as for the United Kingdom. See the changes in GEC page for the change history.
There's a list of them on this United Nations Economic Commission for Europe page. Neil Parker, John Weeks, and Reg Wilson (of Europlate), in their book "Registration Plates of the World" (ISBN 0-9502735-6-2 paperback, 0-9502735-7-0 hardback), list the codes actually being used on vehicles. Parker) is the primary source of the data given in the table above. Faroe Islands changed from FR to FO in the mid-1990s), the old stickers don't disappear in a flash.
However, some of the codes reported there are not the ones in actual use, because some countries are lax in reporting their codes to the U. In recent years, unauthorized stickers have come on the market to promote various tourist areas. Consequently, even direct observation may not be sufficient to establish the correct Distinguishing Sign for a country.
GAUL - GAUL is a relatively new initiative from the FAO (Food and Agriculture Organization), a specialized agency of the U. It aims to codify all the world's statoids and their changes, from about 1990 on.
So far it has developed data for countries, primary, and secondary divisions, including boundary data and entity codes.
The newer GENC standard has retained these variations, but uses ISO-like codes.
IOC - The International Olympics Committee code for Yugoslavia changed from YUG to SCG at about the same time as the ISO Alpha-3 code.
There is also an indication of which entities are or are not independent countries.
ISO - The Maintenance Agency for ISO standard 3166 is supported directly by the central secretariat of the ISO in Geneva.
They were assigned the GEC codes AX and DX, respectively, by FIPS Change Notice 12, dated 2007-06-11.