Adjectival noun is a term that was formerly synonymous with noun adjunct but is now usually used to mean an adjective used as a noun.
Noun adjuncts were traditionally mostly singular (e.g. "trouser press") except when there were lexical restrictions (e.g. "arms race"), but there is a recent trend towards more use of plural ones, especially in UK English. Many of these can also be and/or were originally interpreted and spelled as plural possessives (for example "chemicals' agency", "writers' conference", "Rangers' hockey game") , but they are now often written without the apostrophe although this is criticised by some authorities.
Expressions with plural possessives are increasingly interpreted as compound nouns with a plural noun adjunct even in cases where this is incorrect because the form cannot be interpreted as not being a possessive, e.g. "mens club" for "men's club2.
Sometimes even style guides contradict themselves. The authoritative Chicago Manual of Style (CMOS) says one may freely decide between "participants manual", "participants' manual", and "participant's manual", yet also states that only "guys' apartment" is standard, rejecting "guys apartment" and "guy's apartment".
Fowler's Modern English Usage states in the section POSSESSIVE PUZZLES: 6. Five years' imprisonment, Three weeks' holiday, etc. Years and weeks may be treated as possessives and given an apostrophe or as adjectival nouns without one. The former is perhaps better, as to conform to what is inevitable in the singular — a year's imprisonment, a fortnight's holiday.