avant-garde, avant-gardes, vanguard, vanguards.

contre-maître, contre-maîtres, foreman, foremen.

5. Of the following compounds the nouns take s in both numbers, because they suggest plurality, as:

un, a,
or
des, some
{ essuie-mains, towel, towels = that which dries the hands.
{ cure-dents, toothpick, toothpicks = that which picks the teeth.
{ porte-clefs, turnkey, turnkeys = one who carries keys.

6. Compounds made of invariables, of course, do not change, as:

passe-partout, master key, master keys.
pour-boire, a tip, tips.

§ 7. Nouns having no Plural.

1. Such are the metals, as:

or, gold.
argent, silver.
fer, iron.
plomb, lead.
cuivre, copper.
étain, pewter.