On the use of so-called 'zero genitive', marked by a simple apostrophe in spelling ('), as opposed to the 's genitive, Quirk, Greenbaum, Leech and Svartvik specify in A Comprehensive grammar of the English Language (pp. 320 & 321) that: In addition to its normal use with regular plurals such as boys', the 'zero genitive' is used to avoid repetitive or awkward combinations of sounds in the ...
So many English (probably a majority) take a sort of short cut by pronouncing the 'ss' as 'sh'. Pronunciation is a sequence of shapings of mouth, toungue and teeth and follows (in every language and dialect) its own law: follow the line of least resistance as you pass from one syllable to the next.
This is a style issue, not a matter of objective correctness or incorrectness. Having said that, I note the advice that various style manuals offer. From Words Into Type (1974): Proper names. The possessive form of almost all proper names is formed by adding apostrophe and s to a singular or apostrophe alone to a plural. By this style rule, you would express the plural of Ross as Ross's. From ...
Its uppercase equivalent is two characters instead of one: SS. It was apparently also once used in just the same way English, but I cannot find just exactly when or where. Was it used in manuscripts only, or in printed books too? During what time period would this have run?
Both express possession, of course. We use 's with singular nouns. For example, " my son's toys " will be "the toys that belong to my son". We use only an apostrophe (') after plural nouns that end in -s: " my sons' toys " means that I have more than one son and these are their toys. We use 's for possession with the other plural nouns. For example: " my children's toys; women's wishes, etc.
