Ivan,

Does the The American Heritage Dictionary use CBs or C.B.'s (the second of which would be correct)?

This page quotes from a number of style manuals:

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 (whether spelled out or in numerals) form the plural by adding s alone. [Examples: SOSs, several YMCAs and AYHs, CODs and IOUs, the early 1920s." [14th edition, section 6.16]

MLA [Modern Language Association] Handbook for Writers of Research Papers
"...do not use them [apostrophes] in the plurals of abbreviations or numbers (PhDs, MAs, VCRs, IRAs, 1990s, SATs in the 700s)." [3rd edition, section 2.2.2]

Turabian 'Manual for Writers'
"Form the plurals of most single and multiple capital letters used as nouns by adding s alone: 'The three Rs are taught at the two YMCAs'. [5th edition, section 3.5]

The Gregg Reference Manual
"Capital letters and abbreviations ending with capital letters are pluralized by adding s alone. [Examples:] three Rs, four Cs, five VIPs, six CPUs, CEOs, IQs, PTAs, YWCAs..." [7th edition, section 623]

European Commission Translation Service. English Style Guide
"3.37 Plurals of abbreviations. Plurals of abbreviations (MEPs, OCTs, SMEs, UFOs, VDUs) do not take an apostrophe."

Ed