The MLA Handbook for Writers of Research Papers : "Do not use an apostrophe to form the plural of an abbreviation or a number" and the examples include acronyms such as PhDs and VCRs. The Chicago Manual of Style : "So far as it can be done without confusion, single or multiple letters, hyphenated coinages, and numbers used as nouns... " Among the examples are SOSs, CODs and IOUs.
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