Meaning of -'S in English
suffix denoting the possessive case of singular nouns and of plural nouns not ending in -s (John's book; the book's cover; the children's shoes).
Etymology: OE genit. sing. ending 2.
suffix denoting the plural of a letter or symbol (S's; 8's).
Etymology: as -S(1)
Oxford English vocab. Оксфордский английский словарь. 2012