Trying to work out howcan stand for an organisation who's initials are actually IOS (International Organization for Standardization), a quick bit of Googling came up with the following.
, founded in 1947, is a worldwide federation of national standards bodies from some 100 countries ..... According to , " " is not an abbreviation. It is a word, derived from the Greek isos, meaning "equal", which is the root for the prefix " -" that occurs in a host of terms, such as "isometric" .... The name is used around the world to denote the organization, thus avoiding the assortment of abbreviations that would result from the translation of "International Organization for Standardization" into the different national languages of members. (http://searchdatacenter.techtarget.com/definition/ISO)
So basically, because translating "International Organisation for Standardization" into the 100 different languages, would create up to 100 different initials, they use - but they use it as a WORD not as initials.
So if the Organization that defines the of a film use " " as a word, rather than spell it out as letters ("I.S.O.") does that also mean that when we refer to the of a film, we should use it as a word = " " rather than as intials = "I.S.O."? Because most of the time when I've heard people speak of it in relation to film, they spell it out as individual letters. Which is correct?