NOOOOOOOOOO!!! Just add an “s”.
I genuinely hope this is on-topic: son of a bitch.
For me there’s a significant difference between “sons of a bitch” and “sons of bitches.” I refuse to accept the validity of “son of bitches” and therefore “son of a bitches” just doesn’t compute.
What I really love though is when a woman will refer to her son as a “son of a bitch” as if he had something to do with it.
Carry on…
I think they are “sons of bitches”: if they were “sons of a bitch”, then they would be at least half-brothers.
Stop mocking those of us who were old enough to learn to type on IBM Selectrics. 
(Seriously, I cannot make myself do anything but double space after a period at the end of a sentence.)
I learned to type before the IBM Selectric was put on the market, and I learned to use a double space after full stops.
The question has been disposed of admirably (“trades union” is one thing, not several), so I’ll note that constructions such as attorneys, governors, solicitors, and postmasters general, being legal terms, are based on French constructions which place the adjective second.
To further confuse things, though, you have “lieutenant generals,” not “lieutenants general” – because a lieutenant general is a type of general, not a type of lieutenant.
Major generals and brigadier generals are types of general as well, even if the latter are often referred to as “brigadiers” for short. But sergeant majors confuse things, because they are closer in rank to sergeants than to majors.
Not only that, in some kinds of fonts, it looks better. I realize that everyone says word processing and kerned fonts make it unnecessary, and I realize that some software automatically removes that second space, thus making the sentences run together visually in a style I find irritating…but I still do it anyway. When the software does keep/display that second space, I can see it and appreciate the easier visual separation it gives to ideas in text. I miss it.
Sailboat
I nearly posted “Yep, that’s me,” but since this is a thread about proper usage I must say “Yes, that is I.”
One thing that bothers me about plural acronyms is uncertaintly what to do if the acronym is lower case. It is one thing to write “PINs”, but quite another if you write “pins” for “personal identification numbers”. And how do you pluralize “MIPS”?
This confusion is exactly why some guides say to use the apostrophe. If it’s good enough for the New York Times…
MIPSes? It shows up in a Google search, not that I’m trusting that. 
Yeah. I’d say “Sergeants Major,” as these were originally a type of sergeant, not a type of major.
Nevermind about the plural of Rear Admiral…
That’s a cinch. An abbreviation that is written as a word is an acronym. An acronym works like a regular word, and all regular rules apply. Though I don’t know if PIN has become “pin” yet. I guess it would, since it’s pronounced.
“Nasa’s rockets.”
Or in a contrived question, one might rhetorically ask, “How many Nasas are there?” (Jeez that last one looks weird.)