What is (or are?) the suggested abbreviation(s) for the United States of America in Spanish? I've seen:
(And only the last two actually makes any sense to me!)
In spanish there is a rule for plurals of nouns that involve countries or organizations.
That means that when the word is plural there must be a double initial or a repetition of itself. This means the following:
E.E.U.U. is incorrect because it gives the interpretation that the name is a four worded noun.
The correct forms are the following:
The initials in english (USA) are not accepted in reference to the US.
The rule of the Double initial follows the overall rules of writing of abreviatures in spanish: (According to Rincón Castellano)
Plural de las abreviaturas
And here are the rules for plurals of initials: (According to Rincón del Castellano.)
Plural de las siglas
The rule that applies here is the plural for abreviatures since EEUU is an abreviation.
Now EUA is not permitted and not widely understood in the Hispanic world. EEUU and EE.UU. is more common than the other forms.
EEUUA is acceptable and as per comments not widely used. I for one have never seen it, and I have lived in Colombia and Ecuador. I don't know about Mexico or Argentina. In Spain they use EE.UU.
Further reading: RAE
I always though of the rule of doubling the initials for plural as a little cumbersome, but it's just the way it is.
At least in Argentina, EEUU is the most used, and I've seen it with all the combinations of dots you listed.
The way I see it, there is only but one single Spanish speaking country who shares a border with the U.S. and that is Mexico. Just like Americans abbreviate our country with U.S. for short Mexico by the same turn has taken it's cue from Americans and abbreviated the U.S. as E.U., Estados Unidos.
If you are feeling bent out of shape because the word America is left out, then just think about the fact that there is North America, Central America, and also South America. Those three afore-mentioned comprise The afore-mentioned three are known as simply "The Americas" and do not distinguish U.S.A. as the only "America" in the world! Think how offended either South America or Central America feel about thinking USA has exclusive rights to the word America, in fact, in order of habitation, the E.U. is last in line.
South and Central have a longer track record in terms of inhabitants. Therefore, in being heir to all, coming from a Native/America/Mexican/American/Scottish/American, E.U. is the proper term.