One of the obscure yet elegant rules for acronyms in Spanish is to repeat an initial to indicate a plural noun in the acronym. Such is the case of E.E.U.U. for Estados Unidos (United States) or F.F.C.C. for Ferrocarriles (Trains) lit. ferrous ways.
I have seen this rule applied when only some of the nouns are plural an other nouns are singular, v.gr F.F.C.C.M. the acronym for Ferrocarriles de México.
However I've often seen the abbreviation for Unión de Repúblicas Socialistas Soviéticas as U.R.S.S. instead of U.R.R.S.S.S. as the rule would imply.
Note that a Single U Is used in this case since it refers to a singular union from a plurality of Soviet Socialist Republics, Soviet being a singular adjective to a plurality of Socialist Republics (Soviet Socialist republic being only one of many different kinds of Socialist Republics); hence a Single S for Soviéticas and two S for Socialistas in contrast to E.E.U.U. which refers to a plurality of States (E.E./Estados) that happen to be United (U.U./Unidos)
Is this a formal exception to the rule or is this an example of rule hierarchy for acronyms, say avoiding cacophony taking precedence over the rule of doubling a noun's initial to indicate plurals? Is this a case of simplicity and legibility being preferable to confusion caused by elegant complexity?