All of these seem to represent councils of various trades, not multiple councils of a single one. Salford is essentially part of Manchester. The “Manchester and Salford Trades Council” and the “London Trades Council” are councils representing the trades of their respective areas. This is especially clear in the second example, where it’s pluralised as “trades councils” and “trade_ societies.” Even the last one seems fine to me. It’s a council belonging to several trades.
I suspect that the reason why “Trades” is in the plural, even though it’s functioning as an adjective, is because each council or union represented many trades, not just one trade. Another example is “trades hall”, i.e., the building where those trade(s) unions would meet – that’s used mostly in Australia, but it’s found in Britain too. If you talk about several of those buildings, then you talk about “trades halls”.
I used to go to a school which was variously described as a “Boys High School” and as a “Boys’ High School”: I think both can be defended grammatically, and in both cases the point was that there was more than one boy attending the school. But if you wanted to talk about several such schools, you would talk about “boys high schools” or “boys’ high schools”.
Incidentally, here’s a picture of a very fine trades hall, which is no longer used for that purpose, but is now part of as technical college.
If that was the norm, and the explanation for what’s under discussion, then why did ‘trades union’ not develop as the singular itself?
Two hypotheses:
(1) Many unions would have represented just one trade.
(2) It feels unnatural to have adjectives in the plural in English. To take two extreme cases: “scissors” and “trousers” only occur in the plural as nouns, but as adjectives they drop the “s”, e.g., “scissor blades”, “trouser legs”.
Trades-union and trade-unions are both correct, but they don’t mean exactly the same thing. Let me explain. I’m a long-time member of the UAW. Originally, the A stood for Automobile, but now we include also Agricultural equipment workers and Aerospace workers. Thus, I belong to a trades (more than one trade) union.
The International Brotherhood of Electrical Workers is a union of one trade, a trade union. If you talked about the IBEW and the Machinists’ Union, that’s two trade-unions.
Some larger organizations are many trade unions lumped together, such as the AFL-CIO. So, the AFL-CIO is a trades-union made up of many trade-unions.
Perhaps this has clarified the question, or maybe I have muddied the water.
No, I think that’s pretty much what I meant, anyway!
Almost every day, I drive past “Two Cousin’s Beauty Supply.” I try not to look, but am compelled to read the sign each time. It’s like driving past a bad accident.
Even better (it’s in German, but you’ll get the gist) is the sign next to my office that says “Alle’s für die Katze” which means “Everything for the cat” - you’ll notice the COMPLETE ABSENCE of a plural or a genitive in that sentence - “alles” is just a word that happens to end in “-s”.
One note on the last question of the OP on the evolution of spelling: German spelling used to NOT allow an apostrophe/'s for genitives (contrary to English usage - cp. “Peter’s cat” in English, but “Peters Katze” in German). However, when the German spelling reform came around, they decided that since so many people were doing it wrong anyway, they would now allow it to be used. We call it the “Idiot’s Genitive” in my household.
Thanks folks, the example of ‘brothers-in-law’ has kinda sealed the deal for me. I’d always got that right and never really thought about it.
Now, if i could try to kill the ‘Trade Unions’ debate for the moment and ask another couple of questions.
Nobody has responded to my side question. That always happens to me, is there a one question per thread rule round here or something? 
Kinda related, i’m interested in what people think of this extract from the wiki article “It is common in informal speech to instead pluralize the last word in the manner typical of most English nouns, but in edited prose, the forms given above are preferred.”
Does ‘common’ mean it’s acceptable? Why does the article use the word ‘preferred’ instead of the stronger and more definitive ‘correct’? And to link back to my side question, does the fact that it’s common in speech mean it is likely to become acceptable in written form?
“Common” tends to mean “acceptable in speech and informal writing, except when around jerks.” If you want to sound educated, say “Surgeons-General,” but don’t bother to correct yourself if you say “Surgeon-Generals.”
And yes, if it’s common in speech–and not just in a single dialect–it tends eventually to be adopted in writing, whether we like it or not. Examples are failing me at the moment, but it’s process that’s similar to, if slower than, the adoption of new words into writing. “E-mail” is acceptable in formal writing today, while it probably wasn’t fifteen years ago (or longer–I wasn’t really writing much at six years old).
According to the Samford University Style Manual, it is perfectly acceptable to use “'s” to pluralize some acronyms.
English is a flexible language at times (at others, not). So flex it when you want to.
Yes. They’re wrong*.
I see your Samford University and raise you the Chicago Manual of Style (the online version is subscription-only so you’ll have to trust this reference:
Furthermore, plenty of other US style guides cited here prefer my method. A bonus seems to be that it was written by our very own pinkfreud, unless someone’s using the same name.
*Look, I acknowledge that it’s a stylistic, not a grammatical point, and the US often differs from the UK, but it’s the main cause of most of the damn confusion, IMO, so should be stamped out mercilessly like the dirty little vice that it is. Hence, “wrong”.
I think we’re on the brink of yet more confusion, with cross-Atlantic differences in the use of terms. [ur=http://www.unison.org.uk]Unison, a union for all public-sector workers and the largest in Britain, calls itself a ‘trade union’.
Uuuh, I hate that acronym plural s apostrophe. It just looks wrong. Thank you, jjimm (and by proxy pinkfreud), for providing me with an actual cite that I can use to intimidate my coworkers :D, who stubbornly refuse to leave the darn thing out. (I know, I know, written language in evolution and all that).
This thread seems to have got quite complicated, but the answer to the OP is very simple.
The reason it’s not “webs site” and so on, is that “site” is the main noun, and “web” is a modifier. (What kind of site is it? It’s a site on the web). Same with “cat call” - the important noun is “call”, and “cat” is telling you what sort of call it is.
So, with “secretary general”, “secretary” is the important noun, so that is what takes the “s”. “General” tells you what sort of secretary it is.
“Daughter-in-law”, again, “in-law” is simply a modifier, so the “s” goes on “daughter”.
So it’s easy to see that whoever said the plural of “trade union” is “trades union” is wrong. “Union” is the important noun here. Again, it doesn’t matter that “trade” is or can be a noun – just the same as “web” or “cat” above, it is merely a modifier telling you more about the “union”. The plural is obviously “trade unions”.
“Trade” is certainly a noun, but it is being used attributively and the head noun, the one that has to be plural is “union”, so the only acceptable plural is “trade unions”. Elementary grammar.
Could “trades union” ever be appropriate? If it were a union of many trades, for example? Well, do you say “rat nest” or “rats nest”? I would not say the latter. On the other hand, as Stephen Pinker has pointed in his book, “Words and Rules”, you might say “mice nest” and his explanation for that is the central thesis of his book.
The other ones, like “court martial” is a direct borrowing from French in which adjectives mostly follow the modified noun. Although the French plural would be “courts martials” with both "s"s silent. Why we say “attorney general” rather than “general attorney” I cannot fathom and I really have no objection to “attorney generals”. English nouns have a strong tendency to have “s” plurals and most of the exceptions include the commonest (and therefore most resistant to change) nouns in the language.
I suppose you still use a double space after a full stop? 
I’ll admit I followed the pattern above and said “tablespoonsful” but I think it’s actually tablespoonfuls.