Using your example, the preferred plurals would be IBs or I.B.'s
(however, the latter would be frowned upon by some authorities, since
the periods are not really needed).
"Forming plurals of abbreviations
When forming plurals of abbreviated forms:
Form the plural with a lowercase s (never an es).
PBXs (not PBXes)
If the abbreviation contains internal punctuation, form the plural
with an apostrophe s.
(but CPUs, LANs)
Avoiding periods in abbreviated forms
In general, do not use periods in an abbreviation or acronym unless it
can be confused with a word (such as in for inches)."
Taligent Style Guide
"The formation of plurals and possessives is usually pretty
straightforward, except in the case of abbreviations and acronyms. The
following rules should help.
To form the plural of an abbreviation, a number, or a capital letter
used as a noun, simply add an 's' to the end.
- A group of MPs
- The late 1940s
- Mind your Ps and Qs
To form the plural of an abbreviation with periods, a lowercase letter
used as a noun, and abbreviations or capital letters that would be
ambiguous or confusing if the 's' alone were added, use an apostrophe
and an 's'.
- A group of M.P.'s
- The x's in the equation
- Sending SOS's"
Writer's Block: Plural and Possessive Abbreviations
"Proper Use of the Apostrophe to Form Plurals
An apostrophe is also used to form some plurals, especially the plural
of letters, symbols, and digits.
- Regina received four A's on her report card.
- Timothy used too many &'s in his paper.
- The judges gave the diver two 9's and two 8's.
It is no longer considered necessary or even correct to create the
plural of years or decades or abbreviations with an apostrophe.
- He wrote several novels during the 1930s.
- There are fifteen PhDs on our faculty.
- My sister and I have identical IQs.
(If you wrote Ph.D. with periods, you would add an apostrophe before
the pluralizing 's': Ph.D.'s) If the abbreviation ends in 'S,' it's a
good idea to separate this final 'S' from the pluralizing 's' with an
EditFast Grammar Resource: Apostrophes: Forming Plurals
It should be noted that, in this matter, the New York Times does not
observe the generally-accepted rules:
"Use of plurals is another area of confusion to authors and editors.
As with everything, Chicago/Turabian style takes precedence in this
project. One area of specific confusion when it comes to computer
terms is with acronyms. Most people mistakenly add an apostrophe and
letter 's' to make an acronym plural. The major proponent of this
incorrect method is 'The New York Times,' even though all publishing
houses and computer magazines agree that it is wrong.
The correct method is just to add the letter 's' to the acronym. Here
are some examples:
PBXs (not PBX's)
VLANs (not VLAN's)
NICs (not NIC's)
PCs (not PC's)
The possessive of an acronym plural has the apostrophe after the s.
The only time we accept an 's is when the acronym is separated with
periods. For example, Ph.D.'s."
Computer Dictionary Project
Google search strategy:
Google Web Search: abbreviation plural apostrophe
I hope this clears things up a bit. Basically, it's best to take it
easy with the apostrophes. But if you use 'em and somebody criticizes
your usage, you can always point to the New York Times as your model.