Forming Plurals of Abbreviations
The plural of an abbreviation is usually formed by adding s to the end. However, although not common practice, it is acceptable to use 's if it is deemed to assist the reader.
Examples:
Once I have finished this PC, I will only have 3 PCs to configure.
We have a dozen SUS's to collect from the station.
(SUS - Soldier under Sentence)
(In this example, the writer considers that SUS's is clearer than SUSs and has
opted to use the 's version. This is acceptable.)
The firefighters could not assist with three of the RTAs, because the BA's were still in the stores. 
(RTA - Road Traffic Accident / BA - Breathing Apparatus)
(There is no good reason to write BA's - especially after showing how
abbreviations are pluralised with RTAs.)
Pluralising Abbreviations with Full Stops
(Periods )The
rule above applies to abbreviations with full stops too.
Examples:
Once I have finished this P.C., I will only have 3 P.C.s to configure.
Not one of the S.O.S.'s was received by the coastguard.
Placing the Full Stop
(Period )
A full stop is not required after the s when forming the plural of an abbreviation with full stops.
Examples:

should be M.O.T.s
(garage sign)
Anna was refused credit due to three C.C.J.s. and mortgage arrears. 
(CCJ - County Court Judgment)
We have only done two M.O.T.s since January.
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AVOID THE VERSION WITH AN APOSTROPHE
As covered in this
lesson on apostrophes, 's can be used to show possession. For example:
The MP's photo shoot will be held at 7 o'clock.
(This is the photo shoot of one MP.)
Therefore, using 's to show a plural can lead to ambiguity or, at the least, cause the reader to pause momentarily while a mental check is performed to determine whether the abbreviation is a plural or showing possession.
In summary, you should be aware that the 's version is an option you can employ to assist the reader in spotting an unwieldy abbreviation instantly. However, in all other cases, you should opt for the s version. (Very few grammar books condone the 's version.)
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