How long have we had a consistent distinction between -s, -'s and -s' ?

Andrew Dunbar wrote:

> While I am a big fan of the OED, I don't believe it
> has no prescriptive content at all. For instance look
> at its entry for "philosophers' stone". All quotations
> which have an apostrophe put it before the 's'. Other
> Oxford dictionaries also put it before the 's'. The
> full OED makes an analysis, a judgement, and then
> prescribes only the apostrophe-after-s spelling,
> omitting the other. [...]

--
Anton Sherwood, http://www.ogre.nu/
"How'd ya like to climb this high *without* no mountain?" --Porky Pine